You really only get to decide who you do/do not want to be with and there's nothing negative there. Someone else can't decide for you if a third person can stay with you? No need to point out that you don't like the person or anything else, you just say you're sorry but they can't stay with you. You don't have to give a reason, or you can go down as a "prude" or whatever by saying it's against school rules, etc., other people's opinions about your behavior are not fact, they are just there to help you evaluate your behavior for yourself; other people don't get a vote on what you like/do not like, do/do not do.

I would not tell others that a third person is bad news, I would just say that, for yourself, you do not wish to be at that activity/get together. Say you're going to study or whatever (whatever you are going to do if you don't do what everyone else is going to do); keep it impersonal. But bad mouthing a third person who isn't there for someone else's benefit, that's not a good idea; you don't know what their relationship is like and everyone has to learn their own lessons. That person may behave totally different around the person you are trying to "warn". If you have decided not to be involved, that's all you can do, your good judgment ends when it is no longer about your own needs.