so it's happened a few times aye?
might be time to go without a car for a week, drop the fuel tank out or just divert the filler tube if possible to a tank of something that would KILL a car, if you cant divert the filler tube then add a makeshift one to a tank of car killer, half petrol half diesel will stop em doing it, but might not kill there car.
ok, ok, ok thats one idea that involves a bit of revenge however if your clever and any good with electronics you can wire a switch up so that when your car is locked and the petrol tank gets opened (just like for the bonnet and boot) the alarm will go off.
Hell i'm going to add a petrol cap alarm to my car within the next few weeks, why not.