All you would have to do it put a toggle switch on the fuel pump wire and hide the toggle, or put a toggle on the power to the starter solenoid. Shouldn't be to hard.
I am hoping my ignition cut-off with my Python alarm would take care of that for me. If they don't disarm the alarm with the remote, then it won't start. Is this as fail safe as a kill switch to the fuel pump?
Until the theif figures out how to disable that part of the alarm system.
I like the fuel pump relay thing, because you can stick the relay in your pocket and walk away.
If someone does get in, they need to know how to diagnose a missing relay, know where to look for the relay, and know how to jumper the relay to start the car.
Most Mustang enthusiasts on this site would need a wiring diagram to do this, so I doubt the average theif will put in that effort...
As far as I know the 4 cylinders all had an IRCM (Integrated Relay Control Module), in which the fuel pump relay is housed (it's sealed so don't try to bust it open).
...being this is 2.3 tech, this is why I mentioned simply pulling the FP relay may be tougher than it seems. The 5.0's have a simple relay though.
I dont know if they currently make em, but Clofford alarm systems had a module that would interface with the alarm and had a wireless fuel pump kill. The kill module wired into the fuel pumps wiring, but communicated with the host alarm wirelessly, so thieves couldnt trace wires back to the module and try to defeat it. Even if the host alarm was defeated, the module would not allow the fuel pump to work. Its the most advanced setup i've seen for this application, and once installed, all you have to do is arm/ disarm your system with the remote. If you have the money, thats the route I would go with. There becoming more scarce, but if you search clofford on ebay and then filter it to how only car electronics, you should run across one before too long.