When a post gets old, instead of simply closing the discussion, send comments and trackbacks to the moderation queue. This will greatly reduce spam.
For documentation and tips, please refer to the official plugin page for <a href="http://planetozh.com/blog/archives/2005/02/02/wordpress-plugin-auto-moderate-comments/" title="Auto Moderate Comments" rel="nofollow ugc">Auto Moderate Comments</a>

Ozh' Auto Moderate Comments

When a post gets old, instead of simply closing the discussion, send comments and trackbacks to the moderation queue.

This plugin will list posts spammers find the most attractive.
It will help you identify on which forgotten post you should close comments, and also define potentially spam attractive keywords in your titles (eg. “comments”, “feedback”, “guestbook”…)
Read more: <a href="http://planetozh.com/blog/2010/09/spam-magnet-blog-posts/" title="Spam Magnet Blog Posts" rel="nofollow ugc">Plugin’s page</a>

Ozh' Spam Magnet Checker

Identify posts spammers find the most attractive.

Ozh' Auto Moderate Comments vs Ozh' Spam Magnet Checker