Admin tools: Advanced spam cleaner
This plugin helps admins to find and remove spamming users in Moodle. It supports various methods for spam detections, including akismet apis, which is used by most wordpress sites for spam detection. Also the plugin provides hooks, if you want to implement any other thrid party api, spam detection methods.
PS:- This is a tool to clean spam, it doesn't do any pro-active marking of spam (Yet!) .