I'm going to disallow my ex girlfriend from bringing her new boyfriend to the apartment.
Why? You've already broken up with her, right?
We are still living in the same apartment.
I realize it may be uncomfortable to see your girlfriend with someone new, but you can't control who she brings to your apartment just because you don't like it. She lives there with you and has just as much a right to bring people she wants there.
Well I will keep them from making out in front of me.
You have a right to not see them making out and you can set boundaries what they can do when you're in the room, but I am glad that you are not disallowing her to bring her boyfriend into the apartment anymore. I do hope you can move on and just get a new relationship from other people.