One of the most popular MediaWiki extensions is ConfirmEdit, and with good reason. It comes bundled with MediaWiki, since version 1.18, so you don’t need to download it. You just need to enable it and then configure it.
What is it? In short:
The ConfirmEdit extension lets you use various different CAPTCHA techniques, to try to prevent spambots and other automated tools from editing your wiki, as well as to foil automated login attempts that try to guess passwords.
To enable, just add this to your LocalSettings.php:
wfLoadExtension( ‘ConfirmEdit’ );
Now you need to configure it, as the default configuration is not, in general, terribly helpful. They say there that the most effective CAPTCHA methods are “ReCaptcha NoCaptcha” and “QuestyCaptcha” but note that with QuestyCaptcha, you must make sure that your questions are hard enough to trick the robots, while still being not too difficult for real users. But with questions that are too simple, the robots will get past this tool.