Immobilizers are generally regarded as chips embedded in the key of the vehicle which need to be validated by the vehicle security system in order to start the engine.

Most alarms come with a starter kill which renders the starter inoperative as long as the system is armed.

The simplest method would be a kill switch which kills the fuel pump or starter (as previously posted). My cousins in L.A. did that to their Z3 and Boxster. But instead of hiding the switches, they just put them in plain sight using OEM-looking switches.