Use a cleverly hidden but accessible to you DPST mini toggle switch. Connect two terminals from one side of the switch across the pickup coil wires. Connect the two terminals from the opposite side of the switch between a good ground and the fuel level sender signal wire. When the switch is activated, the ignition won't fire because the pickup coil signal to the ignition module is shunted and null. At the same time, the fuel gauge will indicate Empty. When the switch is deactivated, everything will function normally. The idea is to spoof a would-be thief sufficiently that the normal tricks used to hot-wire wouldn't work such that (s)he would give up on it and look for a different target. Of course, if you want to compound a thief's frustration, you could couple this approach with the 'defeat relay and magnetic switch' earlier suggested.