Larry is right. The surest way to kill the runaway motor is to stop the flow of fuel to the engine. I'd cut or rip off the fuel supply hose. The fuel in the pump will still let the engine run a short while but it WILL stop before too long without a fuel source. The lift pump moves MUCH more volume than the engine can use, and there is a constant, large volume of fuel pumped back through the return lines. Cut the feed, and there shouldn't be a whole lot that can sucked out of the filter, and the IP should be shoving excess out the return. Disconnecting the metal injector lines would be the most positive shutoff though.
Blocking the intake airflow may, or may NOT (as in Larry's case), be effective. Regardless, an ounce of prevention is worth a pound of cure (in this case, several hundred pounds of new engine). All DIY MB mechanics should own a MityVac, and if you just TEST a newly-installed shutoff valve, you won't have this problem!
