What causes a run-away propeller?
Often the overspeed is caused by the airflow forcing the propeller to move faster. It's like braking on the engine while going down a steep hill, even without touching the gas pedal you may speed up and redline the engine. Very problematic if you already overheated your brakes in a 18 wheeler.

If you have a variable pitch prop you can feather it to reduce the effect of the airflow. Or just follow the procedures in the overspeed checklist for the aircraft.