I have had this once in the microlight when a student froze solid on the bar after encountering a ball-tearing thermal at about 400 feet on climb after takeoff.
Seated behind him, all I could do was try to correct the worst of the sudden movement of the control-bar, then shout "Relax! I have control, Relax!" on the intercom whilst I reached forward, physically grasped both his wrists firmly and lifted them upwards, forcing him to let go of the bar.
This succeeded, and I then told him to rest his arms for a few seconds which allowed me to return the microlight to the correct attitude and heading.
He was most apologetic afterwards, and when he had recovered sufficient composure I allowed him to take control of the bar again and the flight proceeded without further drama. He subsequently became a very competent and enthusiastic pilot and is still active.
In this case I believe it was a simple case of sensory overload occasioned by proximity to the ground and a massive disturbance of the control-bar which caused the aircraft to bank to an alarming degree. With all my strength I was able to partially correct this, to the point where I could then reach forward and grasp his wrists and thus force him to relinquish control.
It all happened pretty damn quick, but I do recall being quite forceful in grasping his wrists and using a very authoritative voice on the intercom.