How can you remove a spark plug that's had the head rounded off?

I have just done this on a cast-iron head ..... plug head was rounded off and insulator cracked. The only way was to completely remove the insulator core (much time, cursing and swearing) byand then use an ezi-out to extract the remainder being careful not to let anything fall into the cylinder. Also be sure the piston is at BDC so you don't damage it as well. In hindsight, when it was found tight, I should have left it alone and taken it to a professional. Good luck!