Well as I (naively) suggested whoever owns the actual ship (and thus disposes of it) can claim on an escrow that isn't released until the ship has been verifiable disposed of in an environmentally sound fashion. So if the ship ever is sold to another party, that would be built into the purchase cost (that they could claim this money), even to the final purchaser (the wreckers). Probably some huge loophole or perverse incentive that I haven't thought of, but that's at least one suggestion.