It's a combination of user flags and us burying/killing duplicates. We kill duplicates unless they have an active discussion, in which case we bury them (i.e. demote them in rank while leaving them open).

There are also penalties for flamewars. Originally I turned up the penalty because the discussion was so horrible. Users pointed out that a story of this magnitude needs to be on the front page, which was a good point, so I lightened the penalty.

By "horrible" I mean violating HN's values of intellectual substance and personal civility so badly as to be irremediable. There are some topics the community here has empirically proven itself to be incapable of discussing without that happening. I resisted this view for a long time—ask tptacek or davidw, my adversaries in this debate for years!—but my position either was wrong or at some point became so.

I don't know yet what the long term solution is, but the status quo is untenable, especially because these things aren't static—they get better or worse, and the trend has been worse.