Whatever you do, don't write a virus as that's criminal.
But you could sell all the office stationary on ebay.
Make sure that you denude your work place well in advance of your departure, they could be fussy on the last day, so make sure that all the decent stuff has long gone by then. Such as that nice chair, oil painting, refrigerator, and potted plant.
Don't forget to put chewing gum under the desk of the person who helped you decide to leave.
And finally. This one is courtesy of someone who was part of a mass sacking of an IT department, but missed the "important meeting". Park the Company car in the short term car park at Heathrow, and send the car keys by second class post. Obviously without the ticket, or bay number.
