Presumably that switch was put in because something else wasn't working and not cutting the fuel off.
If you wanted to bodge a work-around, you could replace the switch with a relay, and then take an ignition switched 12v wire from somewhere (like the stereo loom) and connect it to the relay.
Thus when you turn on the ignition, the 12v goes to the relay, flipping the relay, sending power to the fuel cut off solenoid.
Alternatively, just run the ignition switched 12v wire direct to the fuel cut off solenoid. (I did say it'd be a bodge)