Is this something you're worried about happening regularly? A runaway is an emergency situation, it's not normal and is caused serious mechanical problems. This is different from, say, a high rpm hang when you're trying to dial in your fuel pump settings or from when you have a stop solenoid failure in which case automatics had a stop lever and on a manual you stall the vehicle. If you are concerned about an engine runaway, a CO2 fire extinguisher sprayed into the air intake is the old trick, just make sure it's a CO2 type and contains no dry powder.