I rented to college students in groups of 5, and had a "shithead boyfriend/girlfriend" clause in the lease. When we sat down to go through the lease and we hit that provision, I would tell them, "One of you will end up with a shithead girlfriend. You know, the one your friends can't stand. Your parents can't stand. Your roommates can't stand." (They all turn and look at Dave, who says, "Ah, jees, guys, she's not that bad.") If she stays more than 10 days in a month, any 10 days, the rent goes up by a fifth and you all have to pay for her. Now, one characteristic of the shithead girlfriend/boyfriend is they are cuckoos. They want to slip in to the place, and root somebody else out of the nest. The rest of you are the somebody else. When the shithead girlfriend or boyfriend approaches 10 days in a month, or if you just get sick and tired of running into him all the time, you just have to tell your roommate you are about to call me and you're going to make him pay for the extra expense you all incur. It's just a way to help the one with the shithead boyfriend or girlfriend get chordate, because even they get tired of having him around all the time."
Usually at the end of a tenancy I'd ask them if they'd had to invoke the shithead girlfriend clause, and (nearly) all of them would grin and say they'd used it once or twice. The one who didn't grin would say, "Ah, jees, guys, if you'd just gotten to know her...." When four of them grinned and looked at the other, who looked sheepish and finally grinned, you knew at least one bullet had been dodged.