The Inertia Switch in the trunk can be used as a kill switch. You'd need to open the trunk every time, though, to turn it on and off. I used to use it when I went away on long vacations. I would go one step further and instead of pressing the switch off I'd unplug the connector to it to outwit a would-be thief.
In my opinion, the easiest way to install a kill switch for the fuel pump is to wire a switch in series with the inertia switch. Instead of running two wires from the trunk to a convenient point under the dash, the wire to the inertia switch can be accessed at a connector behind the trim panel below the EEC (I think it's called the passenger side kick panel). This is a 10 pin connector with 8 wires on either side (tan/orange, red, red/light blue, red/orange, tan/light green, and red/light blue, and two pink/black wires). You will need to cut the red wire and connect the two ends to your kill switch.
I don't think that a kill switch between the ignition switch and the starter relay would be that effective as the thief will probably try to short the two large terminals on the starter relay (although on a manual transmission he/she would need an accomplice to press the clutch pedal).