fail2ban is an automated firewall tool and this module provides some recipes that allow you to easily firewall spammers or people who otherwise abuse your Drupal site.
The Drupal part of this module requires the core syslog module to be enabled and provides you a with few more options for the syslog settings.
The rest of this module consists of configuration and regular expressions for use with the fail2ban utility and you will need administrative privileges on your server's command line to be able to install and configure these.
If you have any fail2ban recipes you would like included in this module, please open an issue for the Recipes component and attach both the filter and the jail configuration.
The 7.x-1.x and 6.x versions of this module have much less functionality. They were originally written to allow comment moderators (with the correct permissions) to submit comment spammer's IP addresses to fail2ban only.
I do not currently have any plans to port 7.x-2.x functionality to Drupal 6, though the 7.x-2.x fail2ban recipes should work fine on Drupal 6 in most cases.