If its vitreous enamel i.e. glass then unless it is loose and you can pick it out then the only way I know is to soak it in hydrofluoric acid but I would not go anywhere near that stuff, its highly dangerous. It may be possible to break it by tapping it and then picking out the pieces but depends on how well it was applied in the first place.
If its the resin type of enamel, I have burned it out in the past but its very sticky and makes a mess.