Again, a silly and potentially dangerous thing to do.
Just take control, rebrief and hand control back to the student.
When I was a student in 1968, the instructor had a habit of trying to nudge the aircraft into a spiral descent with his knee. I got fed up with this, so one day I applied full opposite lock with both hands on the yoke, giving him a splendid bruise in the process.
Sorry, but there is no excuse whatsoever for interfering with the controls when the student is flying.