careless - water-based pen ink will wipe off with a damp cloth but can, if left a while, leave a tinted residue. Use a proprietary white-board cleaner (Eddings do one). Obtained from stationary merchant or educational supplier (they use a lot of white-boards in schools). Spirit-based ink is more problematic as you have to discover the correct solvent. If you have the offending pen, make a mark on a scrap of glass or plastic, let the ink dry well. Then you have to try a range of solvents to see which one'll work - meths, surgical spirit, white spirit/turps, lighter fuel, petrol, one of which should work (it's more often than not meths). If they all fail then you'll have to, as a last resort, use carbon tetrachloride. Of course there is no guarantee that the board surface won't be damaged by DIY attempts. Maybe a visit to your local janitorial supply company for help - they have vast ranges of all sorts of cleaners & solvents, and experience.