I think the bullying customrs are probably well known, but the procedures are needed so those organizers or volunteers know what to do when one shows up, as M.Ted indicated. You have one person who can deal with the individual away from the group, who's aware it's a waste of time but doesn't mind being the distraction. You listen, try to help, then when the intent to manipulate becomes clear, smile, walk away and let the manipulative person decide what they want to do next.

I've also been the person in the office to whom the difficult people were supposed to be sent. You do what you can, and you learn when to move on.