So in college I lived with 3 dudes in a semi-large 4 bedroom floor. Anyway, one of the guys would always drink our hard booze we left in the freezer and then deny it.
So me and the other two guys bought a bottle of Jager, drank half of it, then filled the other half with laxative and put it in the freezer.
Friday night comes along and we all split for other parties while the 4th roomate stayed and had some friends over to pre-game. We got home around 1:30, and sure enough over half the bottle was gone, and sure enough, he was in the bathroom... him and his friends couldn't get off the john for the entire next day! Vengeance is oh so sweet.
Yes, it did indeed discourage the theft. He was good natured about it and repented. The trick was to put it in booze that had a strong taste to max it, and to freeze it so the taste was even harder to discern. Besides, Jager tastes like crap anyway.