Board Thread:Support Requests - Getting Technical/@comment-5079138-20121219140341/@comment-3335785-20121220223343

Whilst it is an option that can be considered with community discussion and a history of high vandalism, it is very rare to see. I believe it comes under the catch-all clause which stops you doing anything major to the functionality/design of the oasis skin:

"Not to intentionally block, remove, or otherwise obstruct the proper functioning and view of advertisements, and/or user interface and functionality by other users, including but not limited to changing or adding javascript or CSS changes to the Service that would prevent the proper display or function of advertisements and/or user interface and functionality;"

Essentially anonymous users editing is a normal function of Wikia, and as such cannot be changed without consulting them first. I can't see any explicit clause saying Wikia do not offer private wikis, although companies like ShoutWiki do offer this function.