Communicate the influencers more... most people follow leaders, and it would help set the tone for new members.
HN's guidelines should be turned into LightBox style documentation. Put it where it matters right next to the input boxes (and in the case of dupe-checking do it by AJAX and communicate it inline).
Guidelines should encourage the positive, and where something is negative then it should be pointed out that they should attack the argument and not the person.
Have a very small set of very clear rules that are enforced transparently, openly and brutally. The lines will only trip up the worst offenders. If you move these lines, only do so because you've listened to the community and they believe the lines are wrong. I use a system on one of my sites in which I track how much a thread is read, and if a post is flagged by more than a certain number of people then I know the community is saying something got through and the rules either aren't being enforced or might not be right.
Mostly: This is not a technical problem. It's a people problem.
There may be some tools you can build to help, but it is people who will use those tools and it is they that make them effective. Even then, there's only so much you can do without fully empowering others.
One ends up realising that a forum is a microcosm of society, and the owner/leader either chooses a form of government in which the citizens will be happiest and yet the aims of the government are achieved, or the owner/leader delays such a decision whilst the population grows and increasingly friction occurs.