Not at all easy to get rid of, but undiluted concentrated Roundup will do it. (Avoid the normal gardening Roundup - it's already been diluted) The concentrate isn't cheap, and Ebay is probably your best bet. (Do a search for 'roundup concentrate')Don't cut back, but apply to foliage when in full leaf. Use a spray bottle on a large droplet setting to avoid any spray drift, and be sure to cover adjacent plants. Good luck.