The only passenger I had freak was my wife. But of course that was before we were married, when she still would pretend that it was ok. Her freak out was not too bad, it was a bumpy takeoff from grass in a Grob 103. She kept saying "Oh my God, oh my God" funny part is she didn't realize she was saying it out loud. I found myself reaching for the release, then decided the safe thing was to get at least to pattern altitude. Once above the trees when it smoothed out she calmed down. Even with this 'easy' passenger freak out scenario, it adds a lot to your stress and workload. I would not want to have somebody lock on the controls. I would use the fire extinguisher. Better to not have left the ground.
What would you tell them? The truth.
-- IFMU