I could be wrong here, but I suspect that the stain is caused by Tannin. This stuff is notoriously difficult to get rid of. To quote ancient history; when I was in High School, a graduating class spread leaves in the halls as a prank. The tanniin the leaves left brown stains in the flooring that are still there today. I hope that someone here has a silver bullet for you, but I suspect that your looking at a repaint.