If you want to disable the starter, use the purple wire to "S" terminal. It's easiest to grab it at the starter rather than unravel wiring harness under the dash.
Another way to kill the starter is the purple wire at the neutral safety switch, more involved with an automatic, but it's a piece of cake with a manual trans
If you just want to kill the ignition, Use the 12 volt source wire at the distributor for HEI, or use the positive terminal on the coil if you have points.
If you have an electric fuel pump a kill switch for that is a good theft deterent.
I use both fuel and ignition methods (more than one) on a car with a key switch source, so even if the switch is found, it will take a key to operate it
Simple toggle switches just aren't enough in my opinion. Hope that helps.