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.

Disable the Admin Bar that now appears on the public side of WordPress blogs.
For a hassle free integration, drop the plugin file into <code>wp-content/mu-plugins</code> (create the directory if needed) and it will be automatically activated.

Ozh' Auto Moderate Comments vs Disable Admin Bar