I think that if a truly capable and determined thief wants your car, there's no way you can stop them from taking it.
I think the secret to keeping your car safe from carnap is to make your car "less desirable" for a thief to even attempt to take it.
I believe that the longer it seems to take a thief to take(work on) your car, the less desirable it becomes. The thief is concerned about 2 things: how long it takes to get into you car and drive away with it, and how much effort he/she will expend to take your car.
For example: 2 nice cars are parked right next to the other, Car A and Car B. Both car doors are locked. Car B though has an alarm system, complete with blinking lights and what not to let people know that it does have an alarm system.
A potential thief would probably hit Car A instead of Car B. It'll call much less attention to the thiefs actions to work on a car without an alarm.
Using this example, I believe that a combination of passive (your door locks, your steering locks etc.) and active deterrents (motion sensors, car alarms etc.) will serve to make your car "less desirable". For instance, your standard old car alarm + steering wheel locks like "The Club" makes for great carnap deterrents. It will appear to criminal elements that it will both take time and a lot of work to get past the alarm and the steering wheel lock to get your car.
Now on to the topic at hand: ways to disable your car
If you have an aftermarket steering wheel, or are thinking about getting one, Smart Racing Products sells quick release assemblies for steering wheels. I belive that these quick release assemblies are used quite often in racing situations where the steering wheel must be taken out first in order to accomodate the driver. They are pretty expensive though, but I just though I might throw that idea out there for people to consider.
It might look funny though to go to a mall, park your car, and walk around the mall with your steering wheel in hand! :lol: