Does anybody know the easiest method to electronically disable anybody starting your engine even if they have your car keys?

Is there a single fuse in the fusebox which can be removed when you park your car in a less than salubrious part of town?

In the old days, it was easy, one just put a little piece of cardboard in the distributor cap to disable the engine from firing up,

 

There are any number of relays or fuses you could pull. How would they get your keys short of a hijack?

What car?

 

Not 100% sure this will work as I have not tried it.

At the ignition switch you "could" cut the Red/White wire (IP34-5) and splice in a hidden switch. Its the Key sensor for the Passive Anti Theft System. (see schematics fig 2.2). Idea is the Passive Anti Theft System doesn't see it has a key inserted and so never reading the keys chip thus never giving the OK to start.

 

The OP asked a question and hasn't logged on since. Wow.

 

Sorry, I've been offline.

The car in question is 2006 XJ8 VDP.

The problem is to avoid somebody who hijacks your keys from driving away with your car.
This could be simply swiping the keys from your desk when you are not looking (Option a) or even a carjacking in the street (Option B).

For the latter case, you would need a separate electronic switch in your pocket which needs to be "activated" to restore a suitable electrical connection to enable the engine to start.

The question is what is the best "suitable" electrical connection for Option A (static and manual) and Option B (real-time and wireless trip) ?