Blocking it out works, but total removal or blended repainting works best. Paint dries to the touch fairly fast but takes longer to cure. If you act quickly, complete removal may be possible with no further painting required. Washing makes color coverage easier when repainting. The challenge is to remove graffiti without damaging the underlying finish.
Special care is critical on transparent finishes like varnished doors.
Consulting professional assistance may be necessary before risking permanent damage to the surface. Start with the mildest cleaner, working up to stronger ones until you get results. Test small areas first. Try detergent and water Mineral spirits are effective on uncured graffiti. Methanol is effective on uncured graffiti, but will likely dissolve a latex painted surface.