IANAL or anything even remotely resembling one, but it seems to me that this could be begging for legal trouble if an employee was fired for this. I could be very wrong, but depending on how you go about this it has elements of entrapment, etc. all over it.
EDIT: Also seems like a very minor issue. I'm tempted to think that you're trolling. Perhaps this person wasn't aware it was someone else's? Really, I think a memo reminding everyone that their own food is their own should be enough to clear this up.
I can vouch that this isn't a troll question. Having worked with him for a while, he's asking a good question about how best to keep his company family intact without making everyone feel like jerks about the whole thing.
"Person who takes lunch from the fridge" isn't a protected class as far as I know, and a lot of states are basically "at-will", which means you don't really need a reason to fire anybody at any time.
EDIT: Also seems like a very minor issue. I'm tempted to think that you're trolling. Perhaps this person wasn't aware it was someone else's? Really, I think a memo reminding everyone that their own food is their own should be enough to clear this up.