If you want to get back at the thieves then there is a cunning plan. Find a few days when you wont need your scooter, pour a load of sugar in the tank. Sit back and wait for thieves to steal fuel. Change fuel and sit back in the safe knowledge that some "yoof" is sitting at the side of the road scratching his head wondering why his pride and joy has come to a juddering (and expensive) halt.

A bit of time on your part but well worth it.