If you put a swtich in the wire that goes to pin 33 on the red engine ECU connector, when it is open circuit the engine will not start. However, the engine EC already has built-in immobiliser software, so the engine cannot be started without the ignition key, so personally I think that any extra level of security is unncecessary. A thief would either need the key to your particluar car or would have to put the on a trailer, or fit an entirely new engine management system in order to start the engine.