Once it starts to run away on fuel or oil it's pulling it's own fuel through the injection pump and the little plunger in the FSS is not likely going to be able to drop down and stay in place to stop the fuel flow, nor is it likely that the shut down lever on the side of the IP will stop it, but if you catch it just as it starts to run-away it "might" be stopped by these means. So you either need to eliminate the fuel source, like a block off valve in the fuel line and hope it doesn't start eating oil instead to fuel it's hunger for the run-away, or as mentioned the only "true" way to stop it is to eliminate the air flow into the motor.