Try a different herbicide. Crossbow or Finale may work. I would cut the plant back and paint the concentrate on the cuts.
It might take several attempts, but you'll get it.
If all else fails call in a licensed commercial spray contractor. They have access to restricted herbicides that will kill just about anything.
I'll look into those suggestions...I've asked a plant guy before, and he just giggled. Pretty much said to learn to live with it.
Maybe it will get big enough, we can build a treehouse and be the new generation of Swiss Family Robinson.