Here's what I do....just my 2 cents

Idle - if I'm wearing gloves, which I always to when marshaling, even for electrics, I tap the flywheel with my finger. If I'm not wearing gloves, I use my toe.

Runaways - hit it with my shoe. I've got a size 14, so there's little chance of a nitro motor flywheel eating all the way to my toes.

That being said, I also have a T-Maxx that I goof around with. You can't get to the flywheel very easy with fingers or toes, so I keep a 3/4" delrin rod in my pocket when I'm running it, and use it to tap the flywheel.