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.

Personalize, if not humanize, your posts, adding (CSS) popups containings avatars next to mailto links, as well as next to any word. You can chose avatar images, or let Gravatar chose it for you.
For documentation and tips, please refer to the official plugin page for <a href="http://planetozh.com/blog/my-projects/wordpress-plugin-avatar-gravatar-popup/" title="Avatar Popup" rel="nofollow ugc">Avatar Popup</a>

Ozh' Avatar Popup

Add CSS popups next to mailto links or next to any word. Can be any custom image, and has gravatar support.

Ozh' Auto Moderate Comments vs Ozh' Avatar Popup