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.

Guesses your visitor’s Country from his IP. You can then add commenter’s nation flags next to their name in comments, for example.
For documentation and tips, please refer to the official plugin page for <a href="http://planetozh.com/blog/archives/2004/08/27/ip-to-nation-plugin/" title="IP to Nation" rel="nofollow ugc">IP to Nation</a>

Ozh's IP To Nation

Guesses your visitor's Country from his IP

Ozh' Auto Moderate Comments vs Ozh's IP To Nation