despatchers maybe don't have the authority BY LAW to prevent a pilot from leaving if he wants to, BUT how to you react to Stand 22's comment about not removing the airbridge if one was attached? In this situation what choice would the pilot have?
I would suggest that the pilot is not going to put the lives of his passengers and HIS life in danger. Therefore, if the pilot wanted to go, I would let him go - but I would report the matter. If you feel an aircraft is unsafe - then report it immediately to airfield ops - they DO have the power to prevent an a/c from departing.
Issues like these will always crop up, especially if you work for a handling agent and not an airline - the Handling Agent is often "piggy in the middle."