Ok, I know this is old, but there are some good ideas here.
Things I've seen that work great as hidden switches.
Large Stereo plugs. (Just a hole with the plug removed, and all you need to do is wire the stereo connections together to complete the connection.) Friend of mine has his fuel pump power supply wired into one of these, and has the plug on his keychain with a quick disconnect. No plug, and the car is not going anywhere.
Using stock switches. You choose, and you can get really sneaky using things they can clearly see, and would not think of trying. (Like the Defogger switch, or headlight washer switch as a momentary button to start the car...)
Window lock is another mentioned here.
Whatever you decide to do, try and find a wire with low current/amp loads, so your switches last longer, and you don't need a relay. (like if you cut into the clutch start switch so you have to push in the clutch, and hold down a button to get the car to start.... A combination of switches thrown the right way is another one I've seen, but I'll not go into great detail on because it could give away ideas to potential theives. (AKA, Ass sucking bastards.)
I really think the fuel pump is the best. They can crank over the engine, but it will never, ever start, and the car can't be bump started this way either. (Something you could do with a clutch kill switch mod, just roll the car down the hill with the ignition on, in second gear, let out the clutch and off you go.)
Whatever you do, do not post pics of the mods on the net. We are all smart, and can figure out this stuff ourselves.
On the theives note, It would be nice to permently loan car thieves copper jacketed lead in so many ways. If your local PD has no intrest in the thefts, perhaps they might be interested in a few corpses reciently suffering from terminal inertia weapons? They might even dust for prints.. LOL
I don't reccomend citizens arrest, or even shot guns. Bring real weapons, and don't bother "arresting" anyone caught dumping or stripping stolen cars. They used to shoot or hang horse thieves, and I belive we never should have stopped the practice.