Try Fantastic on a cotton rag--don't spray computer, of course. Even a slightly damp rag with some baking soda on it would probably work. It's a mild abrasive.

Sometimes, if you take the same color pen and mark over the mark already there and wipe quickly it will remove it. You can try a Dry Erase Marker as well. This works on Dry Erase boards when someone uses a magic marker on it instead of a dry erase marker.