Hi Comar,
sorry you're experiencing this. The good news is that you pretty much know what the janitor is going to do, so prepare yourself for it. Practice the confrontation in the mirror and do it until you're comfortable. This way, you don't let anger blow up and get in the way. I'm not good at confrontation either, so I have to practice. I've dealt with these things by:
* pick up the phone and pretend you're on a call (there's a button on my phone to set the ringer, and I push that and then pick it up and pretend to talk). Then I say "excuse me, I"m on a call". If that doesn't solve it long term, then I feel more comfortable saying directly: "Please don't interrupt me when I'm on a call or when I"m working".
* The minute he steps foot in the office, say "I'm sorry, I can't talk. I have to work." Then start working.
* Put the trash can outside of the door. Keep it there. He can empty the trash without entering the office.
* Any inappropriate comments, looks, leers, or touches can be met immediately with "NO. That's not okay, please go" or something similar. Have one set go-to phrase that can work under any circumstance and be ready to use it. That's why practice is important. You're afraid of losing your cool and letting your emotion out, so practice!