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.

Simple plugin to never write again WordPress, Word Press or whatever. One word, capital W and P, it’s WordPress!
For documentation and tips, please refer to the official plugin page for <a href="http://planetozh.com/blog/my-projects/plugin-to-correctly-spell-wordpress-not-word-press/" title="Correctly Spell WordPress" rel="nofollow ugc">Correctly Spell WordPress</a>

Ozh' Correctly Spell WordPress

Simple plugin to never write again Wordpress, Word Press or whatever. One word, capital W and P, it's WordPress!

Ozh' Auto Moderate Comments vs Ozh' Correctly Spell WordPress