Rampart

Rampart is a robust anti-spam tool for MODX Revolution. It can be used to help prevent spam registrations and postings on your MODX Revolution site, as well as provides DNS blacklisting. It also supports ban matching via IP ranges, and whitelists to allow in IPs. It requires the PHP extension mcrypt, and the Login Extra version 1.5.2 or later. 

Please view the official documentation here:http://rtfm.modx.com/display/ADDON/Rampart