OP - If the situation continues I suggest the following:-
Buy a small syringe (Amazon, Ebay, chemist etc).
Fill it with food dye I found the green dye worked well when I tried this trick with a colleague .
"Stab" the loaded syringe into plastic containers of twatty colleagues chops, chicken, ham etc and squirt a decent amount of dye into each.
Food will look manky and it will get your message across.
The good thing about this approach is it can be done in seconds just incase someone else wanders into the room. 
And BTW if someone is reading this and recognises my methods - sod you, you shouldn't have spent weeks emptying the works fridge of other peoples lunches out and replacing them with your personal shopping and the food you were taking away for your camping weekend!!!!