Where is the fuel shut off switch in a Ford E-150 van?

There isn't a readily accessible, easily-located fuel shut-off switch in a Ford E-150 van like you might find in some other vehicles. Ford doesn't typically include them as standard equipment. To stop fuel flow, you'd need to address the fuel pump or fuel system electrically (likely by disconnecting the fuel pump relay or fuse). This is generally more involved and requires some mechanical knowledge.