If you don't like this understandably unlikeable person, and you have a mean streak, boy, are you in for some potential fun. Change dorms again. Be sure to tell her exactly why you're jetting. Yeah, that's right, off her. There's no excuse for that kind of behavior.
Masturbation is supposed to be a private thing.
If killing her isn't convenient, just rough her up a bit. If she's bigger than you, use a bat. A friend of mine had a former roommate who was whipping the weasel every night. My friend would pray for sleep, so he wouldn't have to hear Fast Fingers Freddie working it from across their very small room. It was a vicious circle.