There isn't a readily accessible, single "emergency fuel cut-off switch" like you might find on some race cars or heavy machinery in a 2002 Ford Focus Zetec. Fuel delivery is controlled by the fuel pump, which is electrically controlled by the car's computer system. In the event of a crash, the fuel pump will usually shut off as part of the car's safety system.
To stop the flow of fuel in an emergency situation with your 2002 Ford Focus Zetec, your best option would be to disconnect the negative terminal from the battery. This will cut power to the entire car, including the fuel pump. However, remember this disables *all* electrical systems and should only be done as a last resort in an actual emergency.
