The flight instructor reported that the pilot receiving instruction was demonstrating a power-off 180 degree landing. He further reported that as the airplane entered a close-in base to final approach, the airspeed slowed and the stall warning light started to flicker. The flight instructor reported that he stated to the pilot flying, "nose down, nose down," and waited for him to correct. Subsequently, the flight instructor took the flight controls, but as he did so, the airplane entered an aerodynamic stall and touched down hard on the runway. During the landing roll, the flight instructor reported that he lost rudder authority but was able to use differential braking to maintain directional control and taxi off the runway. The right wing sustained substantial damage. The flight instructor reported no preaccident mechanical malfunctions or failures with the airplane that would have precluded normal operation.