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.
Did ever pass to your mind that those silly passenger might have more qualification that you have? Might be there doctors, scientist, CEOs and just because they are not sitting in the front of the aircraft you call them silly?
That's one the reasons why they refuse to fly. Your attitude that their just another piece of freight is not helping their confidence on you and your airline being capable to do guarantee their safety.
As a frequent flier (more than 150 flights per year) the safety record of an airline is the predominant factor in defining my flight schedule.