And how would they get to the pump wires? Cut up the rear seat, or unbolt it from the trunk? then wire in a battery well enough so the car doesn't stall out 10 ft down the road? Sounds time consuming.
Also, why not just bring a 12V battery and power the ecu, bypass the main relay all together? ECU much easier to get to than fuel pump.
You have a good point, but a kill switch will def stop more than your local 8 yr old.
Get another guy, or some decent muscles, and yank it up from the front. Connect with 2 pre-made scotchtaps or spend 5 seconds cutting/stripping with a good wire stripper. Look around, it's been done before. They don't have to be smart, just experienced, or with a 'veteran'.
The main relay supplies power to each injector, the fuel pump, the o2 sensor, and 2 ecu wires. That's 4 wires in 3 spots they have to reconnect power to. Enough distance between them that they'd need at least two, if not three batteries. All from one switch or relay you added. In my car there is only one way to get to and bypass the main relay. Dash removal.
One other thing I like to do is use a 2timer bin-switcher I made with one working crome map and one start/idle map on it. That kill relay flips it to the start/idle bin, which does what it sounds like. It has zero fuel and -6deg timing above 20" Hg and 1500rpm.
THey can re-wire my pump & injectors, but I have 4 tamper proof 1/4"-20 bolts holding my ecu/cover down, which also hides the added wires for the bin switcher wire.