Do nothing it will disappear in time after washes itself
Methylated spirit can also be used. I used it for removing the white markings / writings on my Vitz, and it worked.
razor blade and some low flash point solvents work best.
btw - this is almost common knowledge, every child has written with such pens when a child and got into serious trouble with "ammi jaan"
Go to a professional car painter and ask him to remove it. They have the know how and materials to do it professionally.
