Juan, you could also try ReCAPTCHA module for ConfirmEdit.
https://www.mediawiki.org/wiki/Extension:ConfirmEdit#ReCaptcha
We're using that at Wikia with great results so far.
The basic ConfirmEdit (even with the graphic FancyCaptcha) isn't very hard
to crack, unfortunately.
Good luck!
--
Lucas 'TOR' Garczewski
Community Engineering Manager
WIKIA
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