Don't know what knd of ink you're dealing with, but if it's a permanent marker (magic marker
), you could try (may or may not work) a dry erase marker over top of it.
Sounds weird, but I have had students accidentally use a permanent marker on a dry erase white board, and if you go over it with a dry erase marker, it seems to get into the permanent marker ink and lift it. You can then just wipe it off as if it were dry erase marker. It's the only thing that will take it off.
Try first with a bit of dry erase in a concealed spot to make sure you aren't going from bad to worse.