"All ships shall keep a record of which fuels they have burned. Each time a ship docks in a European port, the log shall be submitted to the customs enforcement

"All ships shall keep a record of which fuels they have burned. Each time a ship docks in a European port, the log shall be submitted to the customs enforcement unit at that port. The enforcement unit shall tally the amount of dirty fuel burned since the ship last docked in Europe. "A ship that has burned X dirty fuel shall be charged $Y Euros per liter burned. If a ship is found to have created fraudulent logs, it shall be charged $Y*10, or seized. There is a reward for reporting such fraud leading to seizure equal to 10% of the forfeiture." Or something like that. As long as the penalty is financially meaningful compared to the income of the ship's engineer, you would quickly start to have trouble getting away with using such fuel.