Yet again I spent almost full working day trying to improve antispam protection on wiki.openvz.org.
Currently 99% of all spam on the web is link spam -- bad guys are adding lots of links to the sites they promote in order to increase page rank values for that sites. In fact they get no profit from that, since in all recent mediawiki installation (including wikipedia itself) all the external links comes with ref="nofollow" attribute. That attribute is respected by Google crawler and other such bots and basically means "ignore this link".
Nevertheless spammers keep inserting trash into such wikis. Recently such activity mostly comes in a form of a user page (or user talk page) creation that looks like, well, a legitimate user page (something along the lines of "My name is John Doe, I'm Java coder, my hobbies are swimming and fishing") but with a blatant plug added ("Here are some cool sites: [1] [2] [3] ...").
For about 2 years or so wiki.openvz.org asks you to login/register in order to edit, so anonymous edits are not allowed. That helps a bit, but still there are bots that performs register, log in and post.
So now I have added another restriction: users that have just registered can not create new pages. Here "just registered" means "registered less than 24 hours ago". Note that such users can still freely edit existing pages. Let's see if it helps.
Oh, almost forgot to say it: looks like captchas don't work at all! Either spammers have good OCR tools, or they hire enough human beings to "manually" decipher those cryptic images.
Currently 99% of all spam on the web is link spam -- bad guys are adding lots of links to the sites they promote in order to increase page rank values for that sites. In fact they get no profit from that, since in all recent mediawiki installation (including wikipedia itself) all the external links comes with ref="nofollow" attribute. That attribute is respected by Google crawler and other such bots and basically means "ignore this link".
Nevertheless spammers keep inserting trash into such wikis. Recently such activity mostly comes in a form of a user page (or user talk page) creation that looks like, well, a legitimate user page (something along the lines of "My name is John Doe, I'm Java coder, my hobbies are swimming and fishing") but with a blatant plug added ("Here are some cool sites: [1] [2] [3] ...").
For about 2 years or so wiki.openvz.org asks you to login/register in order to edit, so anonymous edits are not allowed. That helps a bit, but still there are bots that performs register, log in and post.
So now I have added another restriction: users that have just registered can not create new pages. Here "just registered" means "registered less than 24 hours ago". Note that such users can still freely edit existing pages. Let's see if it helps.
Oh, almost forgot to say it: looks like captchas don't work at all! Either spammers have good OCR tools, or they hire enough human beings to "manually" decipher those cryptic images.


Comments
I think the best solution at the moment is to use some logic captcha like a math captch eg. 12/4 = ??
I've also seen captchas like "Press the red smile with the green border". This are versions where OCR isn't successful.
Maybe you can integrate one of them? I do not know much about mediawiki.
Cu,
Mario
Captchas of the second sort that you mention might work though...
http://www.richgossweiler.com/projects/rotcaptcha/rotcaptcha.pdf
It's from a Google study. I think that is a solution for some time. I'am sorry but I could not find the "Red Smiley" example at the moment :-(
Mario