Yep, those little buggers have really deep roots. The only way I've managed to clear areas for anything long term is by getting out the hammer and chisel. The pressure washer route didn't cut it as their little tubes they live in are ultra tough.
These are the ones I have, not as pretty as yours Mike
Pic taken from reefs.org: http://www.reefs.org/hhfaq/img/hydroids2.jpg
I have a large patch in my nano at the moment and its clearing a wide swath through my zoos. Time to have another whack at it I guess.