The first thing I was told on my Multi Instructor course was, if they don't let go elbow them in the face, but pre warn them before you go that you'll do this if you need to. Whem I came to do the flight test, I said this to the examiner and he said why don't you just flick the end of their nose, they 'll soon let go. So it's up to you, elbow or a flick.