HN is a moderated discussion group that's intended to promote curious discourse. As such, community members may flag submissions and comments (if they have enough karma, and no, I don't remember how much karma you need for each). It's not perfect, but that's the intent. AFAIK, everybody has the ability to downvote. That's how moderation is done.

You can learn more here: https://news.ycombinator.com/newsguidelines.html