I don't know how we stop Poster A from saying something stupid without banning people before they post. All we can do is try to discourage Poster B from responding (which we've been largely unable to do). We are always hopeful that people will act more like adults and not let these threads get out of hand. No one is going to change anyone's mind no matter how long they let the thread go.
Well I've considered it for a couple days to avoid becoming Poster B , but I believe it is unrealistic to anticipate adults remaining silent in the face of something they disagree with (case in point, me ). Even if we could achieve a de facto embargo of Poster A, there is always fear the newbie Poster Z will be taken in. Qui tacet consentit. This is especially true in the blind vacuum of an internet discussion, where we can't guffaw, wink, roll our eyes, sidle away, or change the subject. ALL WE CAN DO IS SHOUT, occasionally.
Unfortunately your wording reinforces the impression some of us fear - that the onus is on Poster B simply because Poster A is incorrigible. I'm not asking for any future crimes banning, just earlier intervention and addressing stupidity and other provocations.