Remove the ceiling tile and place it on a work surface that will not be damaged by bleach or paint.
Mix one part bleach to one part water and thoroughly scrub the stained area. If your ceiling tile is made from an absorbent material, use caution and don't allow the tile to get overly damp. The bleach solution may completely remove the stain.
Paint the ceiling tile with a white primer. Kilz is a popular brand that is used by many for this purpose, but you can use the primer of your choice. This will seal the stain and keep it from seeping back through any subsequent paint jobs. Allow the primer to dry completely, usually for 24 hours.
Paint the ceiling tile to match the rest of tiles if they are not pure white. If you can't match the color exactly, you may need to paint the entire ceiling to match. Allow to dry for an additional 24 hours before replacing.