Reaching over and cutting the engine sounds a lot like throwing a child into the deep end and expecting they'll learn to swim.

Having just soloed five weeks ago I can say that there is a high risk that your reaching over would have convinced me you were going to take the plane away and almost certainly resulted in a crash.

Also, if you cut my engine and I crashed I'd have a hard time not holding you responsible for the damage to may plane.

If any of you other instructors think that's a good idea, let me say that as a recent trainee, I think its not at all a good idea.

If a newbie pilot can't make himself (or herself) put it on the ground, then maybe they aren't ready to land.