The flight instructor reported that after she demonstrated a soft-field takeoff, she directed the student to conduct one. During the takeoff roll, the student maintained excessive back pressure on the control yoke and was told by the instructor to reduce the back pressure. Once the airplane lifted off the runway it started to veer to the left; the instructor again informed the student to reduce back pressure to decrease the pitch, but the student maintained a grip on the controls and held excessive right rudder. The instructor stated, “I have the controls,” but the student “locked up and blocked” them and failed to relinquish the controls. The instructor was able to reduce the pitch, but directional control was lost, and the left wing contacted the runway surface, and the airplane departed the runway and flipped over, which resulted in substantial damage to the wings and airframe. The instructor stated that there were no mechanical malfunctions or failures that would have precluded normal operation.