Been in the same situation more than once. Ended up grabbing the student's hand and physically removing it from the throttle.
I once had a trial flight student (big bloke) panic completely and pull the cc all the way to the aft stop and keep it there. It's the closest I've come to having to clobber a student to make them let go. Having said that, a mate was doing some spin training with an experienced PPL a few years back. PPL sat in a catatonic state, terrified, with full pro-spin controls applied for turn after turn and refused point-blank to hand control back to the instructor mate. With not-many-feet-to-go instructor mate had to whack the student squarely in the face to regain control...
As has been said, one to think about before you go off on your next instructional trip. And sometimes when the other options have run out there's no room for PC niceties...
ST