+1 for a steering lock!
As the old saying goes, prevention is better than cure. A kill switch, immobiliser or tracker only come into play once the scrotes are already in your pride and joy, but having a big yellow steering lock on that is visible from outside will hopefully make them think it's not worth the hassle and move to an easier target somewhere else.
I use a Stoplock Pro Elite and in test reviews it took more than five minutes using hammers, chisels and saws to defeat it. The value is not in the steering lock itself but in the visible deterrent it provides.