possibly try a dry erase marker, works on the boards to take off permanent marker, try a little spot first to make sure it works.
or you car try toothpaste. put it on, let it dry, (about 15 minutes) then try rubbing it out in a clockwise or counter clockwise (for the smart on here). then user a wet sponge to wipe off. these are 2 remedies that have worked for me in the past.
other wise go get some white paint and repaint it.