The ban would be applied by the country where they are registered - the flag of convenience. These are already usually poorer countries that offer lax regulatio

The ban would be applied by the country where they are registered - the flag of convenience. These are already usually poorer countries that offer lax regulations in exchange for the registration fees. The problem would be that if one country enacted a ban, all the ships registered there would just switch to some other country without the ban (or that usually skipped that inspection), and the original country would lose out on a bunch of much-needed cash. It might be more effective to tax the dirty fuel at the source, so that it becomes uneconomical to the ship operators to use the dirty fuel in the first place.