I’ve been involved in some pretty hefty pranking. One involved purchases from the animal sciences department at A&M and the other … well, let’s just say it earned the nickname "poopgate."

In any case, if you can spare the expense, I think it would be better to trick the thief into eating things unknowingly, rather than the "shock and awe" route. For example, line the sandwich with thinly sliced cow eye, powder it with grated pig hoof, switch out half of the bacon bits with dried earthworms – you know, stuff that probably wouldn’t taste like much of anything but no one wants to eat. After you get bored you could present him/her with what has been on the menu.

Alternatively, you could do something to find out who the thief is without them knowing you know. Then, you could carry out seemingly unrelated pranks in the hopes that one day you would hear him/her utter, "Man, ever since I started stealing lunches, my life has totally sucked!"

Oh, and one last thing: If you want this to go on, you have to keep the thief believing they are stealing food from lots of different people (i.e., not the same person over and over).