How would the parent break the cycle?
I made it stop with what I call "Escape Plan Alpha".
My daughter is more of an extrovert than a hikikomori, because her big thing was to go gallivanting to hang out with lowlifes instead of going to school or work. She actually had lowlife enablers willing to pick her up from school where she'd dash out between classes or manipulate a classmate into ditching school. Between the fact she stole most of her college savings out of the account and used it to support a druggie boyfriend, and the fact that there were adults who were willing to lie and cover for her while allowing her to be in their homes instead of in school, she was able to dodge consequences for quite a while.
Here are the details of Escape Plan Alpha.
First, and this happened many months before she reached her legal majority, I told her that her actions had consequences. If she continued to refuse to hold up her end of the deal she'd made with me to get the car (have a job, pay for repairs, attend school, obey traffic laws-- not one of these conditions was kept), I would no longer hold up my end of the deal: I would no longer pay for gas to get her to school. If she was not in a high school or GED program and actually attending classes and putting in a full-time effort studying, I would not be paying her insurance after she turned 18.
Her response was to drop out of another school program to go live with a lowlife friend and take care of the lowlife friend's kids, driving them to school instead of going to school herself. Lowlife friend the same one involved in the piggy parkour incident last year: my daughter decided she was Just Wonderful and that I'd been So Mean to kick her and her six (now seven) little bastards out of my house. Lowlife friend decided it was Just Fine to allow my daughter to be in her home instead of at school.
Second, I told her that if she wanted to live elsewhere she was free to do so, and that because she had dropped out of three different high schools and refused to enroll in or attend a GED class, I no longer considered her a student so my obligation to support her financially was done once she turned 18. With the exception of medical expenses I would be providing no further support at the start of the new year, and would no longer be providing her with money for gas or food. Given that she was only using me as storage, an ATM, and a punching bag I told her she could have continued access to those rooms and eat meals with me only if she signed a rental agreement and did one hour a day of work in lieu of rent. I told her that, besides that, I'd be willing to pay her $x (well above minimum wage) per hour for help with yard work and that I'd apply it to her vehicle insurance or whatever else she wished. I gave her a copy of both agreements to read over.
She responded by ignoring the agreements and continuing with business as usual. When my mother came to visit over the winter holidays to help my daughter celebrate her 18th, my daughter made a big deal of throwing tantrums and treating my mother very badly. I won't bore you with the details except to point out that she finally succeeded in alienating every last member of my family with her ongoing verbal abuse and shenanigans. We celebrated her birthday in style, with a family meal at a very nice restaurant where we also invited her boyfriend. She received, as gifts, a new suit of clothing (she threw a tantrum because I wouldn't buy her another pair of runners to go with the 30+ pairs she already had), some gear related to her favorite football team, and a new case and screen protector for her cellular phone which was the gift from my parents.
Third, I enforced what I said I'd do. She went out of her way to be as big of a brat as possible on the Christmas holiday, which is an entirely separate story, but when she came by late at night after a day of gallivanting to ask for gas money, I asked her to do some yard work in exchange for it. She threw a gigantic tantrum, told me she was leaving for good, and I said: "That actually suits me." She stormed out, slamming the door so hard that the latch broke.
Fourth, on zero day (which she initiated by storming out and announcing she was leaving for good) I made sure she couldn't gain unsupervised access to my home. The next morning, my mother and I woke up early and made a hardware store run. I changed the locks and reprogrammed the access code to the garage door opener, and informed the Neighborhood Watch that she was moving out.
Fifth, I acted instantly to eliminate the legal liability: I contacted my daughter and made an appointment to meet her at the motor vehicles division to have my name removed from the title and registration of her car so that it was transferred completely into her name. I paid for this.
(Background: This was actually the most critical step in the entire process. In my home state, if there's a vehicle registered to your address you can be held legally and financially liable for damage done by the driver of that vehicle. One of the problems I had-- a misstep related to the acquisition of the vehicle-- was that my daughter's name was also on the title so I couldn't just sell it when she started lending it out to her lowlife junkie friends after she'd explicitly promised not to let anyone else behind the wheel. Damage done by some moron I'd never met could therefore result in a successful case against me, and I for one didn't care to lose my 'stache and live under a bridge because of something done by an addled junkie I'd never met.)
Her response to being basically given a car was to bitch me out in front of the cashier when I asked when she'd be by to pick up her belongings. Not a word of thanks, obviously. At this point I didn't expect it from her.
Sixth, I started the purge process. I bought some boxes to pack up the clothes, makeup, and garbage she'd flung all over both her bedrooms, which were trashed.
"You realize it's going to take weeks to wash and sort all of this," my mom told me. I was aware of this, having repeatedly washed and sorted all the clothing in my daughter's second bedroom to organize it, only to have the little twat rummage through it or throw a temper tantrum and mangle it all up.
"Wash? Sort? No, we're just going to stuff things into boxes as we go. She likes things to be messy, and spilled on, and she just loves to have garbage and food crumbs in with her clothing, so that's obviously her system of organization. Let's not mess with it. If two shoes are in separate rooms, just put them in separate boxes."
Packing took a day and a half. I did wrap up the fragile or valuable things and label the box contents. There were about 20 boxes including things she'd already taken to support her couch surfing lifestyle. Most of her valuable items like jewelry were missing. Whether she took them earlier or had sold them for money to gallivant with or to pay her boyfriend's drug debts, I didn't know. Nor did I care.
Seventh, I took two hostages: the rest of my daughter's belongings, and the last of the money I'd set aside for her college savings, which would have been enough for a frugal person renting a room to get by for 5 to 6 months even while paying for car insurance. Having secured them, I contacted two people who bitterly hated my daughter and asked for help moving her furniture and belongings out. They were there, truck and all, with bells on. It took two trips to move her bedroom furniture, all the boxes, the microwave, and the other belongings I'd bought her. There wasn't quite enough to furnish an apartment but more than enough to furnish a room for rent. We moved it all into the storage facility.
Eighth, after the move was complete and my accomplices had made their escape, I called my daughter and told her I'd rented a storage facility, and would release the last of her college money in exchange for her assuming the storage contract.
Ninth, I met her at the storage facility, gave her the key, showed her where her stuff was boxed up, and pointed out where the fragile or semi-valuable items like pictures were packed. I told her that the large, heavy mirror didn't make it because it was damaged when we tried to get the smeared makeup off of it. That was an exaggeration. The mirror is alive and well at my home; my mom was able to restore it and get it looking mostly like it did before although there is still damage.
Tenth, I told her she had a week to get car insurance, and that after that point it would be cancelled. I provided her with the agent's card and told her that because of her tickets and her at-fault accident her rates would be pretty high but that she had the right to find cheaper insurance elsewhere. She would have the money to do so for several months.
Finally, we went to the bank and I gave up the high-value hostage, transferring every cent into my daughter's account.
Since then, the Venomous Spaz Beast and I lived happily ever after although I'm still having psychological and emotional fallout from years of living with a deeply disturbed and violent person who was physically, verbally, emotionally, and financially abusive to me but who was entitled to not only my ongoing financial support (required under the terms of my adoption) but 24x7 access to my home. My father came to visit about a month after my mom left and helped me replace two doors, fill in the holes my daughter had bashed in the walls, and supervise the replacement of the carpet and flooring she'd destroyed. During that time my daughter came by in tears: she'd blown through the last of her college savings in less than a month, and could she move back in?
I told her no. She left without much of a tantrum. Three weeks later, she had a job. I wish I could say she kept it, but she didn't: she's living off of a boyfriend's grandparents at the moment after a series of Yes-But fiascos.
Since then she's asked twice more to move back in, I've said no every time particularly since some of the social abuse was ongoing: she was continuing to tell people lies about how I'd abused her and kicked her out, in order to get more from them, and some of them were confronting me about it to try to pressure me into taking her back in. I'd kept receipts from Escape Plan Alpha, so I courteously told the flying monkeys that they were welcome to support her themselves, but that I'd already paid for her education and her vehicle plus providing her with bedroom furniture and enough clothing for six young women, and I could prove it. After defusing three different sets of flying monkeys, and after spelling out the exact reasons why I wasn't taking her back in, I usually hear from her only when she wants something from me. At these times she's polite, kind, and considerate. At long last, I have the same level of respect that she shows to a complete stranger. Sometimes when she's upset she's tried to scream at me, but I shut that down by telling her that not one problem she has can be solved by throwing a temper tantrum, and that if she wants my help she needs to act like a civilized adult. That stopped the tantrum in its tracks, because-- as I suspected all along-- she's always been fully aware of, and in control of, everything she's done.
I'm not being abused anymore and my home is gradually resembling something that has never been the lair of a tantrum artist. The VSB and I are living comfortably, my finances have partially recovered although I continue to pay for my daughter's medical care, and my daughter-- who quit her job-- is now taking a GED class at the local community college and actually showing up for most of the school days. She's still several terms away from passing the GED.
My daughter continues to make drama but it no longer affects me financially. Although she assured me that Boyfriend was really taking care of her and was "a good provider", what she means is that he takes her to restaurants and arcades a lot. He doesn't provide, for example, anything resembling car insurance and astoundingly not one cent of the money she made working went for car insurance either. (She of course had money to buy a couple exotic pets for Boyfriend and to pay a relative's sizable electric bill.) This has cost her: while driving uninsured, she backed into a parked car and the owner of the car called the police. She called me asking me to commit insurance fraud by buying insurance for her car and pretending it had existed before the accident. (I told her "no"). I'm not sure exactly how that ended up, but she's not incarcerated at the moment.
(edited to fix copy and paste errors)