Some are trolls and/or flamewar-stokers while others are substantive contributors. That's a problem with any forum with a low barrier to entry. Users can help by flagging the trollish and flamewar posts. To flag a comment, click on its timestamp to go to its page, then click the 'flag' link at the top. (There's a small karma threshold before flag links appear.)
We sometimes close threads to new accounts when the situation is overwhelming, but I wouldn't want to do that in a case like this. Generally on HN, we try to err on the side of privileging positive contributions rather than filtering out negative ones, and we rely on community moderation and moderator moderation to try to dampen the latter. It only works partially, but it's better than punishing the positive contributors, and definitely better than being a closed community.
We sometimes close threads to new accounts when the situation is overwhelming, but I wouldn't want to do that in a case like this. Generally on HN, we try to err on the side of privileging positive contributions rather than filtering out negative ones, and we rely on community moderation and moderator moderation to try to dampen the latter. It only works partially, but it's better than punishing the positive contributors, and definitely better than being a closed community.