If you have self-registration enabled and no Captcha on your site, you might be getting a lot of spam accounts. Here’s a tool from Ankit Agarwal called the Advanced Spam Cleaner which could help you clean up your site’s spam accounts (proactive protection is coming soon),
This plugin helps admins to find and remove spamming users in Moodle. It supports various methods for spam detection, 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 third party API, spam detection methods.
Read more at https://moodle.org/plugins/view.php?plugin=tool_advancedspamcleaner. This add-on is available for Moodle 2.3 and up.
