I believe Mr. Bean had a door lock idea. a A padlock through the door panel to the back body panel.
Seriously. If you have an electric fuel pump, put a mechanical on/off switch in to supply the power to the pump. Just don't forget to turn it on before you drive away. haha. This idea was from an old railroad mechanic I had the priviledge of working with. Yes, he had one in his cars. A 1930's Deusenburg J model, and a 1970's Oldsmobile 98.
