To: Office All
Subj: To whoever stole my cheese from the Office fridge
Dear whoever stole me cheese from the office fridge,
I hope you enjoyed my cheese. I was looking forward to eating it for lunch, and I certainly hope the joy you gained from stealing it surpassed my annoyance at having it stolen. You will be happy to know that I have replaced the cheese, along with several other food items, and you're welcome to have at them any time you please. You should probably be aware that I have coated a random selection of these items with an extremely powerful emetic that will induce immediate vomiting, loss of bladder control and uncontrollable defecation.
Have a nice day.
This is a 
I really, REALLY hate when people steal my food from the fridge in work. It's the worst thing you can possibly do to a workmate.
Your not supposed to warn them about the poison, that's all part of the delightful surprise.