I suggest considering a plug-in for automatic site moderation and spam protection. The service employs ways of spam protection, invisible for the visitors and there is no CAPTCHA!
How it works: the plug-in redirects a message (registration in the site) from the site visitor to CleanTalk moderation server, then the service checks the message and the sender according to many aspects and gives a response whether to post the message or to mark it as spam, in case of doubt the comment will be redirected to manual moderation.
The service determines the relevance of the message to the topic and estimates the message conformity to the topic of the article. There are developed modules, standard for installation in the site, for popular CMSJoomla, phpBB, WordPress, DataLifeEngine, IPBoard, vBulletin. API may be used for other CMS.
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