The bitterest compound (for humans) ...

... turns out to be Denatonium. So if somebody keeps stealing your food from the communal fridge, try to season it a little with this unique ingredient [1]. Adding some of it to the saltshaker might result in peculiar/amusing situations as well [2].

[1] We tried everything last year to put off our special friend. However, even laxatives and nasty letters pleading to the thief's social pride (people from Singapore are sometimes so naive) showed no effect.
[2] I doubt anyone would notice here at my halls. Dinner is rubbish most of the days anyway.