Use a 1/8" carbon gouging rod with the sides wrapped in electrical tape to prevent it from arcing to the sides of the hole. Use a 12v battery charger or similar current-limited power supply and with great patience, peck away at the carbide until it''s eroded. If you only use a couple of amps, it shouldn't make a big enough arc to damage the hole. And just manage the heat by pecking at it for a bit, then letting it cool. Some people will recommend a water or oil bath for this process, but I'm not 100% sure if it's necessary.