Here is a Taiwanese company that makes a locking fuel shutoff:
Sobo gas lock . I am surprised these things aren't easier to find. My brother has one on his '72 Camaro. It was installed back in the late 70s, though, and I have no idea who made it.
Even better than a fuel lock would be a brake line lock:
Brakelock . Even if a thief got the vehicle started, he wouldn't be able to move it. Would make it a bit harder to tow, too.
