Posting this stuff is like writing the "Let's steal a MK3 manual V1.1."
Stop posting your "exact" ideas, and things like the stock start switches. We all should know where they are, and how they work, no need to inform the masses...
So, with that said, just pick your choice of critical system, and figure out a simple way to deny power to that system, wire up your "switch" or circut and your done. (And as IJ notes, you hope the theif is in a hurry, and does not take the time to figure out your devious open circut....)
That site with the car killing the bad guys... LOL that's dang funny, but the serious truth is that in South Africa for example, car jacking is such a problem, they actually sell active ways to maim or otherwise take out the person trying to car jack you. (From spring loaded swing bars mounted under the car that are released at the push of a button, and would come out and break your shins, to fire spouting nozzles under the rockers to flame thrower the would be car jacker... )