Wanda, it sounds like your ng is doing the best it can with this person. On the ngs and lists I've been on, arguing and debating with people who sound like the one you are describing tends to increase their number of posts. Short, simple, unemotional, polite replies to their posts can clarify the "facts" for newcomers while avoiding drawn-out debates and flaming that accomplishes little more than creating turmoil on the ng or list.
IMHO, a newsgroup or list is a small community with its own unique personality and dynamics. Whatever that personality is, in the long run it tends to prevail over the influence of a single person.