4. Laxative in my snacks
Actually had a co-worker tell me of when he did this to disclose a snack-stealer at this old workplace. Turned out to be a little Filipino lady that no one suspected.
No more snacks were stolen afterwards.
A friend of mine in the Army went one better. His lunch was being stolen repeatedly. He kept snakes at home (boas, pythons, etc) and fed them "pinkies" which are euthanized baby rats. One day he made up a fake lunch with a submarine sandwich in it. You've probably guessed it - pinkies on rye with lots of mayo. The top sergeant had a bad day after sampling it.
I felt bad after we found our snack stealer. He hadn't missed a day of work in 8 years. Next morning he called in sick.