Control grabbers are an even worse menace in a tandem. Many years ago I had a bad experience with an instructor in a PA18 on floats. He had a habit of grabbing the controls, starting a turn, then releasing them without any kind of handover of control. The first time it led to real confusion about who was in control. Between flights, I told him in no uncertain terms if he did that again, I would walk away, course unfinished. It did not result in a personality change, and he continued to think he was God's gift to aviation, but at least the control grabbing stopped.