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.

Have a blog where readers often ask the same questions ? Give the Auto Responder a try and save your support time !
For documentation and tips, please refer to the official plugin page for <a href="http://planetozh.com/blog/my-projects/wordpress-comment-faq-auto-responder/" title="FAQ Auto Responder" rel="nofollow ugc">FAQ Auto Responder</a>

Ozh' FAQ Auto Responder

Have a blog where readers often ask the same questions ? Give the Auto Responder a try and save support time!

Ozh' Auto Moderate Comments vs Ozh' FAQ Auto Responder