Having a plan of what to do with a 'student frozen on the controls' is something every instructor will need at some point. Relying on luck or waiting until it happens before thinking of something is usually not as effective...
True indeed. Quick war story (not real war, of course) During my QFI course at Central Flying School at RAAF East Sale in the 1950's we were on Wirraways. I held the rank of Flight Sergeant while all the CFS instructors were officers. At 8000 ft over West Sale as a student instructor I had pattered Wirraway spinning to my CFS instructor Flight Lieutenant Denis Smallbones (RAF exchange instructor who flew Spitfires in WW2).
He took control and became the student pilot while I practiced to be the instructor. I told him to conduct an 8 turn spin to the left which he did. In the Wirraway the student was in the front seat and the instructor in the rear seat. At the 8th turn I directed him to recover. But the bugger kept on spinning. I raised my voice and said in a firm voice "Recover now, SIR"
Smallbones: "I can't Sir - I am frightened"
Self: "That's OK old chap - I have control now - just let go of the controls.
Smallbones: "Can't Sir, we will all die"
I was unable to free the controls from his grip. By now we were down to 3000 ft from 8000 ft and spinning hard with the idiot in the front frozen with full pro-spin controls jammed hard over.
Self (getting alarmed by now, that he wasn't kidding) "Relax old chap I HAVE CONTROL"
Smallbones" I'm scared Sir" (he wasn't the only one by now)
Self: "Let go of the bloody controls, you fu@*%# ing idiot, SIR".
Immediately the controls became free and I was able to complete the recovery pulling out by 2000ft.
"That's the way, Centaurus" said a chuckling Smallbones. "If you get caught with a student frozen on the controls, don't be afraid to swear long and loudly at him which should shock him into letting go.
Lesson learned; although fortunately that situation never occurred to me in my life as a flying instructor. That last resort technique is not taught at civilian flying schools; but should be, IMHO.