hello,
I've installed a captcha system (same as some Wordrpress or Drupal) on my personal wiki.
There are still some caching problems (well, at least my Firefox caches the image instead of reloading them which is a problem here)
Only my "Talk" pages are editable by non-sysop. Just create an account and try it:
http://www.fxparlant.net/Talk:Captcha
Just reload the page when the imagecode is not readable
François