Centarus, good story! when I underwent my first training to qualify to fly the public in a glider, the instructor in the front seat pretending to be an innocent member of the public pulled the stick right back and stepped hard on the left rudder.
I said "Let go!" politely. And then when we still spun down, I used the same tone of voice and volume that I do for my kids when they better behave or there will be BIG TROUBLE! Which did the trick, he let go and I recovered.
I was quite prepared to hit him over the head with violence. Almost had to follow this plan later on when a trial lesson grabbed the stick in panic at low level on the aerotow. Fortunately shouting usually works.