Shower problem: Can't turn off hot water
I bet you it's the rubber washer that is gone, crushed beyond sealing. Easy and cheap repair. Of they have been replaced and it still leaks, then the brass seat is bad and could possibly be polished/filed to a smooth surface again. If that's not possible then a new valve assembly will be needed. The rubber washer is at the end of the shaft, screwed in with a screw in the center. You could also try a thicker one or a fine shaped one of it will fit.