My preferred method:
The red part is a removable key. It's not a very sophisticated key, but unless someone is familiar with these switches they won't easily by-pass it.
Note though that if you simply put this between the battery and everything else that the computer will have to go through the re-learn process every time you use it. Will also delete any volatile memory (radio pre-sets, etc.) unless you provide them with power.
One way to do this and avoid those issues is to interrupt battery power to the starter only. One truck that I know has done this strictly for anti-theft has the switch carefully hidden in the fenderwell area.