I have no good user-level suggestions for this. From a programming point of view, CMS systems that support comments seem to have relatively good anti-spam options. Drupal has HoneyPot which has been very successful for us. And WordPress has dozens. AVH First Defense against spam seems to work well for me. Maybe incorporating some of their techniques into a plugin for Moodle would help? Though I'm not sure moodle modules have deep enough access to hook into self-registration pages.
Security and privacy
Captcha alternatives?
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