Thanks guys for your contribution.
Idnan has kindly sent me a DIY, 'how to add switch to power supply of coilpack'.
This is valid for petrol cars and done on 335i. If you wish to try on another petrol car then you have to open the wiring box (locatd on Left hand side under the bonnet) and disconnet the wire 8 of green plug. If you car does not start after this, you can extend the wires to add a switcth to this and hide switch somewhere in your car.
should be fine as you can open / close the circuit with keys. So if by chance, somebody locates your switch then they should not be able to close the circuit to turn on your car.
If you are going for a tracker mentioned earlier in my first post, then it has fuel cut supply. You can simply connect that to this and in theory that should not let anybody start your car.
Or If you are is stolen then by a simple click you can open the circuit and your car will halt.
Here is brief DIY how to add switch. (Thanks to Idnan)
I took some pictures of the cut off switch wiring on my car. I left out pics of how to get to the ECU box as that is in a few guides on this forum. Also I didn't take pics of the switch itself as it would give away the location
Green connector at front of box. Coilpack power feed is number 8 on the plug. I snipped it a few inches from the connector and soldered the two black wires onto them:
How to feed wires into the passenger footwell. Push wires through this reach from inside. The hole is at the rear of the ECU box:
How to remove the panel under glovebox, two torx screws and slide it towards you:
And where the wires come through:
Add a switch in the cabin and you are good to go.