How To Beat the Siphoners

I have a Mopar locking gas cap... if you press really hard, it'll engage and spin off without the key. Deterrent, but not foolproof.

Consider they may also just put a screwdriver through your tank if they want it that bad. If I caught somebody hitting my daily driver, I'd happily burn the car to the ground just to watch them spew fire while their lungs roasted. Doubt I'd be able to catch 'em though :-(