Our ABS rings are press on right? I don't know. If you can get the ring off then drill it out that would be best, but I think most times they tend to break when removing.

Maybe apply some heat if you have a propane torch, and then grab the head with some vice grips and hope it doesn't snap off!

What about trying the next size up allen wrench? I believe in those smalle sizes they sell allen wrenches in half sizes. Might be able to cram one in and get it turning.