While Ex-Lax and the rest of the suggestions here are fun, here's what you need to do.
Next time you go out to eat at a decent place, intentionally order more than you can eat. Box the leftovers in a styrofoam take-out container. Apparently it's important to your co-worker that it's food from a restaurant.
Make sure it's an Italian restaurant. This is important to him.
Save the receipt, and leave it on top of the container when you put it in the fridge at work. Apparently he likes to know what his lunch cost you.Last, one word: Visine. Not too much , a few drops will give him a scary case of the squirts. Too many drops will give him diarrhea so bad he'll be in the hospital from dehydration. The dose, and therefore the severity of the punishment, is up to you.
Of course, since you already have another calzone, the weapon of your co-worker's colon mass destruction is at hand.