Modern lawn mowers have a kill switch -- a device that automatically turns off the lawn mower when you press it in case of an emergency. Older lawn mowers might not have a kill switch, but you can still turn them off quickly if need be. A throttle supplies gas to the mower which powers the lawn mower. If you stop the supply of fuel with the throttle, the lawn mower will stop.
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Stop pushing the lawn mower. Make sure you are standing still and the mower is on level ground.
Pull the throttle to the "Stop" position.
Locate the spark plug wire on the lawn mower. It should travel from the throttle to the motor. Pull the wire out of its socket and the mower will stop and won't be able to start again until the wire is plugged back in.