If what you are saying is that the threaded end is all that remains in the head (no ceramic left at all) you may be able to install a screw extractor into the hollow threaded shell. The most effective type are the ones marketed by Snap-on. They consist of a set of round bars with ribs running the length--this is the part that gets inserted into the "drilled to specific size" broken bolt-stud-or spark plug in your case. A hex "nut" (with grooves to match the ribs on the bar) slips over the bar. This is what you apply a wrench or socket to. These sets are expensive so you may need to have this job done by someone who has the proper size kit. I think I'd give the remains of the plug a good direct whack with a flat ended bar to hopefully "loosen" the threads. The plug may have been crossthreaded--or maybe it's a too long of a reach plug whose threads were left exposed inside the combustion chamber and has a coating of carbon now (common problem with air cooled VW Beetle motors that used a short reach plug but had the wrong plugs installed).
I definetely feel for you. The '80 245 I bought 10 years ago had 123k miles. I decided to treat it to a new set of plugs---1,2 and 3 came out no problem--4 turned a few degrees and locked up. I didn't want to deal with stripped threads or a broken plug so I left it alone--and put the other three back in to keep it company. It's working on reaching 270k now. In a worst case scenerio you may need to change the head (not such a bad job and maybe less work than repairing what you have). -- Dave