There are various kinds of shut off switches. For example, the most rudimentary is a fuel cut off switch that interrupts power to the 12v source to the fuel pump. Flip the switch and the car dies in a few seconds, if the car is off the car will crank but it unlikely have enough fuel in the lines to start. This is the most inconspicuous as it can be tiny. I had one under my seat in my first car (88 honda civic) to prevent it from being stolen. Easiest security modification ever because you can do it with RadioShack parts for under $5
A better one that is easy to do is a switch that interrupts the negative from the battery. From the chassis it goes to the 1 of the ports behind the switch, then from the second port it goes back to the battery. This will prevent the car from starting but it will not "kill" the car if it's already running.
The best is one that is used in auto racing (designed required by sanctioning bodies) that has 4 ports in the back, 2 for positive and the other 2 to kill the ignition. This switch will fully shut off the car even if it's running and of course prevent it from starting if it's not flipped. Its design makes it bigger and it's harder to hide. That's where you get creative with it.