If it's "3-to-GO, 1-to-say-NO", who has the authority to say GO?  

IS 135 when you have people that aren't paid by the A/C Operator, they're customers!

And passengers. But, some operators require that customer/passengers have a veto on operations- they can't (ordinarily) make the flight happen but they can stop a flight operation based on company policy.

This can be a serious problem for the PIC when a particular crew objects to certain flights based on other than operational issues... say, desiring a better nights' sleep. Good management can see an obvious pattern when that is so, but it's a good idea for a pilot to keep a record of circumstances that result in lost flights. I'm not going to identify the pilot, crew or program, but a certain crew combination liked an uninterrupted night for extra-curricular activity...