jeez, you must have pulled real hard !!! only the thread is left in the head ? try to drill away some of the porcelain and then cut a thread inside the remains (left thread!) put a bolt in it and unscrew the plug.
you drill an undersize hole in the sheared bolt (or in this case sparkplug). And then use an outsize hexagonal metal rod (this is called an easy out and come in various diameters). Tap this into the undersize hole so its an interference fit, and then use a spanner usually, or other such tool to turn the easy out.Thus enabling the sheared bolt/sparkplug to unscrew from the thread.