Flight Instructor reported a runway excursion during landing rollout due to the student's failure to relinquish control to the instructor. The Instructor secured the engine which caused the aircraft to stop on the grass with no damage to aircraft or property.

Narrative

A private pilot and myself were conducting a training flight to check him out in a Beechcraft Musketeer. He had previously done all of his flight training in a C-172; and had the goal of getting checked out in a low wing aircraft for rent. I was acting as the flight instructor supervising for the flight. The pilot and I had flown for an hour on the previous day; with no unordinary events. We worked on maneuvers and did a couple landings. The student was able to keep the aircraft straight and under control on all landing rolls prior to the event.The runway excursion occurred on the students last landing of the training event. The touchdown was safely executed until the student let out all the back pressure on the yoke. I had anticipated the action and caught the yoke to protect the nose wheel; when the student proceeded to lock up the right brake. We began to drift towards the right side of the runway; while I urged him to relax off the brake to allow the aircraft to track straight. As I said 'My plane;' the student locked up on the rudder pedals and yanked the yoke to full left aileron. This resulted in the right wing 'flying' us 180 degrees and facing us towards the left side of the runway. I was able to wrestle the right wing back to the ground as we entered the grass and pulled the mixture to kill the engine. The student wouldn't relinquish controls and continued to lock the brakes and freeze on the throttle all while I was calling for control of the airplane. We ran into the grass and slid to a stop with both brakes locked up.I secured the aircraft and shut it down; and we both exited the aircraft safely. There was ultimately no damage to the aircraft or to property. The event was a result of the private pilot failing to relinquish control of the aircraft. Instead of releasing the locked brake; he continued to drag the brake and locked up the other; while trying to 'steer' away from the right side of the runway. The private pilot was also around the age of 60; which seemed to delay his reactions to my instructions. Additionally; I had just taught the student to use differential rudder pressure; as he was stomping rudder inputs as needed. I believe this may have led to him landing without his heels on the floor; factoring to the dragging of the initial right brake.I believe this event was the result of human reaction. The best prevention is to better brief and practice positive exchange of flight controls; and remind students to keep their heels on the floor during landing.