A killswitch is typically used for saftey, but I suppose you could use it for theft deterrance. There are two types of killswitches; fixed and removable.
Ignited (
www.ignitedperformance.com) makes really cool fixed style. They light up and all that. The only problem with Fixed is that you can't take the killkey with you. This means that you have to mount it in a very discreet location if you want to use it as theft deterrance.
Ignited also makes a removable style, but it's not nearly as flashy (or expensive) as their fixed style. Alot of people make a removable style, but I can only find an ugly red plastic one. The advantage to this is that you can take the key with you when you leave your vehicle.
You just hook your starter/ignition power to the killswitch and viola. The choice is yours.