Firstly, the passenger's cyclic is removable, and I think it's always a good idea in the R22 to remove the spare set of controls when flying with pax. There's little enough room anyway, without your pax not being able to stretch their legs because of the pedals, and having a collective on either side of them in narrow seats. Secondly, I've had this happen. I was a fairly new PPL(H), with only about my second passenger. He knocked the cyclic quite hard, and it disconcerted me, but no more. I really can't imagine an accident happening this way. If one has...well, freak accidents are possible. As for letting go of the cyclic, as vfrpilot says, you just don't. When you fly an R22, the cyclic is an extension of your right hand.