I had a room mate like this once. I finally got fed up with his antics and snapped a picture of him the next time I walked in to see him naked, jacking off while sitting on my couch. Without a damn towel! (shudders) When he realized I had just taken a picture of what he was doing he freaked out. I told him that if I walked into the house and he was doing that outside of his bed room or the bath room again, I would print a copy of the picture and mail it to his mother. He never did it again.