Do what I did.
I had an awful roommate my freshmen year. Not only did he regularly have sex in my bed, I walked in on him looking at porno on my laptop (when oddly, he had one himself), stored beer in my fridge when he knew there were random checks and if I was found with my alcohol I'd be kicked out of my program, was an all around dick, etc, etc.
Leave in the middle of the night, and take everything of yours - in my case I lucked out because everything nice in the dorm (lighting, TV, video games, fridge, shelves) were mine. I filed the proper paperwork to leave, and even though they required me to get a signature from my roommate tell her to tell them the reason she is leaving BECAUSE of her roommates, and that they would never sign it for some reason - chances are they'll let it slide without the roomie's signature. Residence Life will set her up with another dorm room. They wake up, and her - and all of her stuff that the roommates used - are gone.
This avoids the dirtiness of pranks. Issue is it assumes your girlfriend has some nice stuff that the roommates use. Also Beef Bullion showers are great, I remember that one from bandcamp.