The best cure is time and exposure to normal washing and weathering, it will soon be gone unless it is in a very sheltered location. I am still frequently amazed to see folk filling or decanting diesel on deck without taking the simple precaution of heeling a bucket of water over the whole area first.
As Quandary says.. prevention is better than cure.
Before fuelling I always slosh around the area with a sponge dunked in sea water. Then (I hope) should there be any fuel spashes they'll just float on the layer of sea water and not even touch the decks.