Marker pens are notoriously hard to seal. The best thing to try is a leafing aluminium paint, in other words a paints that has a very shiny silver look, not the metallic aluminium look. Some of the spray can manufacturers have this silver and it will seal the marker pen.
The metallic silver used in these paints floats to the surface and the aluminium platelets (leaves) overlap and seal anything underneath. Also excellent for sealing water stains in plasterboard, ie in ceilings.