Bad Behavior

Bad Behavior is able to even block the spam from ever being posted in the first place.

Instead of merely looking at the content of potential spam, Bad Behavior analyzes the delivery method as well as the software the spammer is using. In this way, Bad Behavior can stop spam attacks even when nobody has ever seen the particular spam before.

Installation:

Unpack and upload it to the /wp-content/plugins/ directory.
Activate the plugin through the 'Plugins' menu in WordPress.

Requirements:

• WordPress 3.1 or higher