The fuel pump is mechanical. However, you should be able to put a switch (relay, not just a switch) in the MAPS line. Without MAPS data the ECU should shut the car down. On the D40 you could use either the MAFS or the FRPS, either would provide zero response to the ECU and the ECU should call it quits to protect the engine.
Isolating the starter won't help, you could clutch it.
Relay operation is simple: you need a reasonably heavy positive and negative wire running from the battery area to the engine itself, so I'd run it along the battery negative cable and then up to the sensor you're knocking out. The positive cable goes to battery positive with a fuse. The negative cable runs in to the cabin to a hidden switch, the other pole of the switch is connected to the bodywork under the dash. Then just cut the sensor wire and hook it up to pins 30 and 87 of the relay (doesn't matter which way). Hook up the positive and negative lines to pins 85 and 86 (doesn't matter which way). The important part is getting the power to pins 85/86 and the sensor wires to pins 30/87.
And if your car ever stops, check the relay/fuse first.