I basically inform them of their legal obligation as my passenger to keep their hands and feet free of the controls at all times unless otherwise instructed. One of my pilot friends is quite fidgety so I let him do most the radio work as it gives him an outlet for his nervous energy, but 99% of the time my pilot friends are quiet and just enjoy being a passenger when flying in the right seat.
I find the non-pilots are normally white as a sheet and far too terrified to touch anything. Although, as my girlfriend says, it could be because they've experienced my driving and the reality has just sunk in that I'm about to take them flying.
