Half a Slice
Tip: if you're going to steal someone's bread, you might as well just take an entire slice. It's not like the person who the bread originally belonged to is going to eat the other half. In fact, they might write a nasty message on it in black magic marker. You could have just eaten it!
Don't be afraid to use your food as a passive aggressive weapon. You can bet the half-slice thief learned his lesson here.
Use Religion Against Them
If someone is stealing your food—or, god help them, your Diet Coke—one of the best weapons at your disposal is to use religion. Putting a nice big picture of an accusatory Jesus will make the heathens think twice before nipping one of your beverages.
Label Your Food As Things People Would Never Eat
This is an extreme measure, but it might be just enough to put off even the most daring office food thief. Label your food as things people don't even want to touch, let alone eat. For example, put your sandwiches in baggies with the biohazard symbol on them. Think about it—if you were going to steal food, would you take that one? Even you were pretty sure it was a joke? Nope, you'd move right on to Todd's leftover Chinese.
Pictured above is an ingenious note. H1N1 (swine flu) will scare off anyone.
Seriously? An entire carton of eggs? Who does that?
Anyway, the person who wrote this note to the egg-thief has the right idea: threaten to expose the employee that's making off with your grub. Nobody wants to get busted by Human Resources for eating twelve eggs.