I think the best you can do in news.arc is to experiment with various forms of throttling (i.e., the first link/comment vote = 1, the second slightly less than one, and so on). Beyond that, it might require you to play the role of benevolent dictator and kill user accounts that consistently engage in nasty behavior. The most extreme option would be to shutdown HN and spawn a small number of child HN-style sites, each with a narrower focus.