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.

A new trend in spam is to duplicate legit comments from your readers to hopefully pass spam filters. Spammers post content that was previously approved on your site, but with their own name and URL. This plugin prevents this.

Ozh' No Duplicate Comments

Prevents spammers from duplicating legit comments but with their commenter name and URL

Ozh' Auto Moderate Comments vs Ozh' No Duplicate Comments