The best option is to use a throttle return spring. In a lot of cases, you'll need a higher torque throttle servo to handle the extra strain of overcoming the return springs tension. Using a "kill switch" of sorts isn't realistic because (as has been said) the air or fuel needs to be cut off and almost anything that will do that will come with its own problems of adding complexity and parts to an already compact package. Gas engines are different because you just need to kill the spark to stop the engine.