Not necessarily true. I've had unruly passengers removed from my aircraft while chocked and with the door open, much to the exasperation of the CSR involved. We may have zero authority according to our manuals, but nowhere does it say we have to take the flight if we are uncomfortable with the situation. I've threatened to walk off and let them find another Captain a few times. It's a very effective method to get things resolved quickly when all else has failed.