What worked for my mom when Tubby peed on my Dad's side of the bed...twice...was she put some plastic over the bed spread. Covered the bed with plastic. I've heard that aluminum foil works well also. The theory behind this is that kitty doesn't like the feel of the plastic or foil on his/her little tootsies, so he/she stays off.

Another thing you could do is close your roommates door so kitty can't get in there. There are also products you can buy at PetSmart and such that are cat repellents. I'm not sure how well any of them work though. Hopefully someone else will have some advice on those.