Oleg, I know it's a little late and you don't have time to impliment a new system, but have you seen slashdot's moderation system? You throw a few karma points to the best posters, and they get moderation abilities off and on, at random. They moderate good posters up, trolls down, and the good posters eventually get the chance to moderate themselves.
Then there's a system designed to catch good posters who aren't good moderators (meta-mods). It seems to work out well.
I believe that the code is available under GPL or something.
I really do second this. Slashdot has an insane number of users. It coined the phrase "slashdotting a server". (If your server was slashdotted, you'd know. As there'd be the higher pitch WHIRRRRR, followed by a loud CLICK, then silence in the server room...) Anyways. Users do get mod points, and it's implemented in various ways and each post is rated -1 to 5. 1 being the norm, -1 for obvious trolls, and 5 being for excellent posts. Thing is, you are free to set your browser for any level. If you were masochistic, you could set your browser for -1 and see all posts by the lovely troll population. I typically set mine to 2, to weed out all the redundent messages.
Best system I've seen on the net thus far. Self-moderating, with your choice of how much garbage you want to deal with. No possible censorship arguments, nor overzealous mods.
As the experience with my third forum showed, the quality of participants is THE major factor. It trumps all others combined. So tweaking rules will not help unless the quality of the folks using the forum is high. It's that simple.
I don't care to belong to a club that accepts people like me as members.
- Groucho Marx