Honestly, and this came from an old school instructor who was an old man and kind of fragile. The issue is that even if you were to strike a student in the headset they might keep thier leg locked on the rudder. What you need is something that draws attention to thier legs AND causes them to release the controls. This old timers method was a slightly longer then normal push thumb tack. If you push it into the thigh of the student not only will the let go of the controls but thier leg will jerk and move based on simple reflexes. The man said he had only used it twice in close to 22,000 hours of dual given but he truly felt it saved his students and his own life both times.

P.S. He kept it attached to his clip board tied with a break away knot. Ask a boyscout how to tie one because I have no clue.