My dad had a friend that constantly bragged about the gas mileage in his new Mercedes.Dad started secretly adding gas to his friend's tank.
That was a partial plot line in a 1972 movie "Pete and Tillie" starring Carol Burnett and the deceased Walter Matthau. Only the car was an orange VW IIRC.
http://www.imdb.com/title/tt0069080/ . I had thought about doing this also. I have a friend with a RAM PU and he needs an adjustment.
I also like the one about being an hour late using the clock as an excuse, mind if I use it?
Other options:
Throw it overboard - Who needs time on a boat?
Use it as a decoration, tell everyone it's broken.
Don't reset it. See how much time you lose or pick up over a season.
Set it to 5 O'clock and remove the battery.