Try the Vulcan 'Death Grip' as practised by Spock on StarTrek!
Seriously, having had a similar experience with a trial lesson in an R22, it made me think hard about what to do in future - the advice I was given was to twist their ear. It is very painful and will shock them, making them instinctively let go of the collective. You then have their attention. Also, it does not injure them in the same way as a puncture wound from a biro in the thigh.....
