Some spammers seem to be targeting MediaWiki installations (even if the wiki uses the nofollow attribute), and altering the wiki 'about' pages or 'Community Portal' pages. These are pages which are linked to in the menu bar (in the default mediawiki installation) but often the pages are never used for legitimate content (especially on smaller wikis), hence the spam has a good chance of going unnoticed despite being prominently linked to.
Other pages which often get spammed on mediawikis include 'Help:Contents', 'Current events', 'Wiki:General disclaimer'.
Media wiki administrators who want to set up a really simple wiki, should take away the links from the side bar which point to 'Community Portal' and 'Current Events'. These imply that the wiki has a large community or is trying to attract one. On many fledgling wikis they seem a bit unnecessary.
To change links in the side bar…
If you are running MediaWiki 1.5. or later: Simply enter "MediaWiki:Sidebar" in the URL where the page title goes. Otherwise click 'Special Pages'->'System messages' and there should be a link to 'sidebar'.
If you are an older version of MediaWiki. You'll need to assign a new value to the $wgNavigationLinks configuration variable. As with all such variables, you can find the default value set in 'includes/DefaultSettings.php' but you shouldn't really change that file. Instead take that as an example. Copy and paste it into 'LocalSettings.php' (e.g. at the bottom of the file). Remove the unnecessary links in this array assignment. Oh and think about upgrading!
Then there's the links at the bottom 'General Disclaimer' and 'About'. These should normally be useful pages to create. Just whack some info in there, check you don't have spam on there already (e.g. CSSHiddenSpam lower down) and then set the pages to 'protected'. Go to the discussion pages for each of those, and set them to 'protected' too. (That's if you can't be bothered to watch recent changes carefully)
On the newest versions of MediaWiki there may be a way to disallow creation of new pages. If you only ever intend have a single page wiki (missing out on the power of wikis really), this could be good permissions option to set. how to do it?