How to use a captcha to allow asks/answers to new users to avoid spam

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Thank you for your answer, dharrah, but I didn't find that option. Finally, I've disabled the feature "Allow posting before logging in", and I changed the "Content moderation method" to "pre-moderate watcher users and audit flagged posts" and the users's status to approved. That way when a new user try to post, this post is blocked until a moderator accepts it. Last step is mark the post and the user such as approved and that way only new users will be moderated to avoid spam. To avoid the moderation of a determinate user you only have to change the user's status to "approved", and to block it (if it's spam) you only have to change the user's status to "blocked".

I hope my answer helps someone and if someone finds another better way (easy, please) I'll be glad to listen to. Thanks.