There are times when you just want to get rid of a plant. I have some little rose bushes that have the worst thorns. I used this method and they we gone in a few weeks.
Salt! Salt will kill about any plant. I suggest that you only use this on plants that you want to kill and make sure there are no other plants around that you want to keep.
1. Dig down cutting the roots of the plant you want to kill.
2. Cut back all branches as close to the grown as possible
3. Pour a lot of salt directly into the wounds. Try not to get much on the soil just on the plant.
I do this on a very warm day letting the sun work on the salt.
This should kill that stubborn plant.