I've had huge trouble over the last year or two with spammers (over on balloonboard.org).
Captchas can work quite well, but are of course a major impediment to blind users, and many are also cracked, and if they aren;t then they are very hard to read for people too. Textchas are generally a better solution (natural language questions that relevant people will know the answers to), and have reduced what was a huge spam problem to absolutely zero when I added it to my moinmoin wiki.
In fact almost any non-standard change to the wiki form elements will stop most bots.
I don't know if mediawiki provides textchas, but if it does I'd recommend enabling them rather than captchas.
Captchas can work quite well, but are of course a major impediment to blind users, and many are also cracked, and if they aren;t then they are very hard to read for people too. Textchas are generally a better solution (natural language questions that relevant people will know the answers to), and have reduced what was a huge spam problem to absolutely zero when I added it to my moinmoin wiki.
In fact almost any non-standard change to the wiki form elements will stop most bots.
I don't know if mediawiki provides textchas, but if it does I'd recommend enabling them rather than captchas.