This is indeed a tricky situation... I'm a Senior at Miami U. in Ohio and I've seen/heard my share of such things.
The best roommate advice I can give is to be sure that you speak openly with your roommate about your feelings. I know that unspoken thoughts will just fester inside and can create other tensions when otherwise there would be no problem.
One thing to do would be to say that you had a specific reason that you didn't like *anybody* sleeping over, rather than just targeting one person in particular. That could really backfire though if later you had a friend you wanted to have sleep over. Alternatively, you could say that you prefer guests not stay on a regular basis because it makes you uncomfortable, or reserve overnight guests for out of town guests. Perhaps that would be your best bet. The girl from down the hall really has no reason to stay in your room if she has her own bed. Although, maybe there is a reason that girl leaves... does she have a roommate with a boyfriend, for example?
I guess I'm rambling now, just trying to imagine different situations, that's all. I hope that helps at least some.