Like he said, a razor blade. Buy a razor scraper at the auto parts store for a few bucks. The surface doesn't need to look perfect like a mirror, it just needs to be flat and free from crap. You won't remove discolored spots in the metal without having the block resurfaced at a machine shop, which is the best way to do it, but isn't required.
Don't use any kind of file or abrasive paper or similar. They will ruin the flatness of the surface and will cause a blown headgasket.