Make sure the whiteboard surface is a proper dry-erase surface. You should be able to wipe away most marks with a damp cloth. If the mark won’t come off with that, try drawing over the stubborn stain with a fresh dry erase marker. Once drawn over, you should be able to wipe it clean with a cloth. Note that the marker you use for cleaning will pick up some of the color you drawing over, so use a darker marker to clean if possible.