There are several ways to "sneaky" anti-theft some in plain sight
Guys used to advocate wiring the cig. lighter to the tach wire. Push in the lighter--if it's clean--grounds the coil NEG. Does not work very well on MSD LOL
If you have any other relays in the car, for ignition circuit, etc, same deal, interrupt the coil circuit.
You can ALSO buy electric fuel cutoff solenoids (for mechanical pumps)