Simply tell them that if one gets off, you're all off and the flights cancelled.See how peer pressure works amongst the SLF and leave them to decide.I don't have the time to be held to ransom by silly passengers.
One of the most irresponsible comments I've ever seen.You,as skipper,do your damndest to explain the problem,diffuse the tension,and convince them its safe.If they still want to get off,that is their prerogative.