You can 'flag' stories with 500 karma, which is basically a super-downvote in the sense that a flag suggests the story doesn't belong on HN at all. My sense is

You can 'flag' stories with 500 karma, which is basically a super-downvote in the sense that a flag suggests the story doesn't belong on HN at all. My sense is that it is a much stronger signal than a comment downvote.