Grapefruit juice
During my first few weeks in halls in uni, I gradually became aware that items left our shared fridge were possibly considered communal property , and were being devoured at an alarming rate. As there were 22 people sharing one fridge, identifying the perpetrator was not easy - everyone swore blind they weren't nicking others' stuff.

In particular, I couldn't leave orange juice in the fridge without someone else emptying it - as my culinary skills were virtually non-existent, orange juice represented almost all of my fruit/vegetable intake, so with scurvy a real possibility steps had to be taken.

I took a freshly opened carton of orange juice and dissolved a good half pound of salt in it. The vitamin thief identified themselves within the hour - there was a strangled sort of retching noise, swiftly followed by a torrent of insults directed at me.

Apparently he'd poured himself a large glass and taken a swig, and immediately realised it was very, very un-orange-like.

"Ah, must be grapefruit juice", he thought, and drained the glass - apparently felt sick for days.
(Photon wasting your time since, Wed 19 Dec 2007, 2:00, 3 replies)

Juice revenge
Serves him right.
(TDub Nunc id vides, nunc ne vides on, Wed 19 Dec 2007, 3:27, closed)

Tame
Sleeping tablets and laxatives gives better results.
(oneinthepink is at your cervix m'lady, Wed 19 Dec 2007, 14:54, closed)