From b2evolution manual
This manual page is outdated. NEW PAGE: Akismet plugin in b2evolution v5+.
The Akismet plugin for b2evolution checks feedback through a central service (http://akismet.com) against spaminess.
You'll need a Wordpress API key, which you can get by signing in on http://wordpress.com/.
The plugin requires b2evolution version 1.8 or later.
The Akismet plugin's source is hosted in the PluginsRepository at: http://evocms-plugins.svn.sourceforge.net/viewvc/evocms-plugins/akismet_plugin/.
You can download it at https://sourceforge.net/project/showfiles.php?group_id=160495&package_id=196866.
 SERVER array
List of $_SERVER keys, that should additionally get sent with the comment data. The default should be safe to not include sensitive data.
(in development) The Akismet plugin now also protects the message form.