I've heard duct tape, coffee grounds, and dishwasher powder, but I couldn't tell you from experience.

I CAN tell you that we smoked in our dorms all the time. Campus police were called twice. One guy got a paraphernalia ticket (and got kicked out of the dorm). Two guys got tickets that said "smoking marijuana" and had to take some bullshit drug class with a bunch of people who got caught drinking at football games.