Given the nature of the situation, wearing a mask is a safety measure. And it should be written in the contract of carriage. Failure to comply with it shall be treated equally to any other safety offence, e.g. not wearing a seat belt when the sign is on. Therefore, it should give the crew the right to issue a Captain's written warning in case of noncompliance. And the follow-up could be a place on the black list with no right of refund for unused future bookings.