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 fun plugin just replaces the “Excerpt” meta-box with something more meaningful for the Modern Redditor.
Oh, also, it’s a neat example of custom string replacement using a filter over gettext 🙂
<h3>Support</h3>
On <a href="https://github.com/ozh/too-long-didnt-read" rel="nofollow ugc">Github</a>. I don’t read WordPress support forums.

Ozh' Auto Moderate Comments vs TL;DR