Believe it or not, alchohol will remove it. Not the rubbing kind, the drinking kind. And I guess that liquor would be a better term.
Try a small dab of Jim Beam or some other whiskey.
No, this is not a joke. I had a friend, and when we started to play a new campaign, he pulled out a battlemat that had been in storage for a few years. Black Vis-a-Vis was all over it, and he took a paper towel, soaked it, and squeezed it out over the board. He let it set for a few minutes, and then wiped it up. We've used that trick several times since then, since it seems that some colors are much worse than others.