Put a relay in the line between the ignition switch start post and the starter solenoid. The output from this relay would supply 12V to the start solenoid in start and supply a ground when not. A screw driver across the solenoid BAT post and start post would give the would-be thief, nothing but sparks.
This relay could be wired to only be energized by the ign switch, thus prolonging the life of the ign switch. The 12V source for the relay-to-solenoid could be picked up from a source closer to the battery.
A lot of the after market alarm systems have a "solenoid Interupt" feature such as this. They even add a delay feature so that if the car is broken into the ignition is disabled by the "Interupt" for 6-10 minutes. Even if they had/found the ignition key it won't start.
