What to put in margerine as revenge?

[fo3nix]

I'm currently in a flat at uni and share a kitchen. Someone (I know who) is stealing everyone's food, and doesn't care. As such, I want to get revenge.

We've tried reporting him and asking him nicely not to, but he won't stop.

One of the things he's often taking is my margerine (and leaving all his little bits of toast at the top too); what can I put in it to stop him from doing it again and teach him a lesson?

I thought about a laxative, but that's mostly bright colours. What would fit well in with Flora?

(We got the idea because a friend of ours did the same to his ketchup -- using super hot chilli sauce.)

[knobbynonuts]

A mousetrap glued underneath the lid.

[BRIGHT SPARK]

Next time youre in the bathroom remember to top up his aftershave.......... well it is a similar colour and sometimes smells better. Good luck.