Soak his toothbrush in liquid pepper, like Insanity sauce....also hit some of his food, something he drinks and mouthwash if he uses it. Hit him with that sh*t randomly for a week - he will get the point. I had a roommate in the Army that tried to prank me all the time....two weeks of Insanity sauce treatment was enough for him to call a truce. I was relentless with it and used the entire bottle....nothing was safe, he told me later that he was so paranoid in our room that he wanted to request a move. He had a hard time pooping for a week after the 'treatment'. Never had to deal with his sh*t again.