My first thought was to let my girlfriend take care of them for a while. She kills every plant I give her. :)

If they're confined to a small area, I would cut them back to the ground and cover them with heavy black plastic sheeting. Smother them.

If they've already spread all over the place, you may have to use a herbicide. Chemicals like Roundup work best when the plant is actively growing. Between now and next Spring dig up as much as you can and try to get all the roots out of the ground. Anything left that tries to grow, spray them with Roundup.

It takes persistence, but it can be done.

Cut it at an angle. Paint the cut with Round-up. It's a goner!