An additional security alarm - which wire should I disconnect to unable start the engine?

It is not that simple; the security system is integrated into the CAN of the car. I can give you the security wiring diagrams so you can see how the system works and what I am talking about. You can try disconnecting wires to see what happens, but this can disrupt the operation program and cause the car to need to be reprogrammed. Check out the diagrams (below). Please let us know what happens.

So, you want to interrupt a wire to a switch for added security. I got you, there is a dark blue with orange wire coming from the power distribution center under the hood. To the fuel pump I would cut and install a switch. Check out the diagrams (below). Please let us know if you need anything else to get the problem fixed.

It is hard to say if what you are trying to do will work, the car runs on a specific program, but you never know. Be careful not to short anything out. Here are the complete wiring diagrams so you can see how the system works. Check out the diagrams (below). Please let us know what you find.

They will be inside the wiring harness connecting to the fuel pump module.

It doesn't say, if you think this is the problem run a new wire from both points, this is a common wiring repair, let me know what happens.

Yep, those junctions are somewhere in the harness they don't say where, you must remove the covering to find them.