This plant can cause contact dermatitis in some people which is what you have been experiencing. You will need to cut it down as far as possible to the base and then either put down some chemical to kill the roots or dig them out completely to remove it, they may have spread out quite a bit. Make sure you wear gloves, long sleeves and possibly a face mask when working on it. You could also employ someone to remove it for you, it doesn't affect everyone, Good luck