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Alternatively, you could let people downvote as much as they want but cap the karma hit at -10.

Comment posts do have the flag, but only if you look at the comment on its own page, e.g., your comment: http://news.ycombinator.com/item?id=234897

You've just exposed one of my objections against this type of limit: it's arbitrary. -5? -10? -15?... I don't like arbitrary limits if we can do without.