Where is the fuel shut off switch on a 1996 Ford E-150?

There isn't a readily accessible, easily-located fuel shutoff switch on a 1996 Ford E-150 like you might find on some other vehicles. Ford didn't typically include them as standard equipment on vans of that era. To stop fuel flow, you'd need to address the fuel pump relay or the fuel pump itself (though working on the fuel pump is significantly more involved and should only be attempted by someone familiar with automotive fuel systems).