In high school, someone would regularly steal my lunch from my locker (it was easily opened). One day, I crushed up several laxitives and put them on a tuna-salad sandwich (to mask the taste of the bitter medicine).
I waited in the lunchroom and watched as the perp consumed the whole sandwich. Then, I informed him of what he'd just eaten. Sweet revenge.
I didn't care that he kept stealing my lunch (or the crappy sweater I made in home-ec)... because he spent a glorious afternoon in the bathroom.