A long time ago I was playing in an event when my opponent had a tricky choice. He was better but I hoped he wouldn't see the strongest move in the position (which I had spotted). His task was made considerably easier when a spectator simply told him to play that move, which he then did. Complaints to the arbiter were met with a 'What can I do?' response, especially as my opponent claimed he was just about to play that exact move anyway.
I had a similar (but not identical) issue today. It was in a non-rated blitz event, and in this case the spectator was a player who had already finished his game in that round. Removing the interfering player from the event was an easy decision. But how to deal with the board situation was not. In the end I borrowed a ruling from Contract Bridge, where I forbade the 'lucky' player from playing the move in question, but allowing them to play any other move. They went on to win the game, so I guess no harm was done.
The other thought I had was to walk back 2 half moves, and allow the 'sinned against' player to change their previous move. I don't know if that helps, especially if the choice was forced, but it may be a way of penalise either side for the actions of a third.

