I have never had anyone freeze on the controls. In the last few months I have met aerobatic instructors who had a trainee apply full rudder and then freeze on the controls. Both cases Extra 300. The first case the trainee appeared to be passed out, holding full rudder. The instructor was thinking he was going to need to bail out and leave the trainee. The trainee recovered from whatever happened and they? landed safely.
My first consideration was to jettison the canopy and see if that would make the trainee let go. Second was to pull about 9 G and put the trainee to sleep. I inadvertently did that once with the 11 yo son of the owner of a two seat Sukhoi. He wanted to see a vertical roll and I pulled a bit too hard. I watched his head flop over to the side. As soon as he recovered he wanted to try it again.?
A typical flight for me in the Sukhoi was plus 11G minus 8.5.?