Wikimedia Bugzilla is closed!

You can stop a page being created by using transclusion and cascading protection:
* Create a page for this purpose and protect it, with cascading protection enabled.
* Transclude any pages you don't want to be able to create, by adding them to
this page, using {{:Page name}}. 

This will show up as a red link on the page, and the page will be protected from
creation by anons (or non-sysops, depending on protection level).

If you attempt this for an image page, however (by entering {{:Image:name}}
then, for some reason, the link is purple and the image page is not protected.

You can see an example of this at
http://www.mediawiki.org/wiki/Project:Protected_titles/list

This may have something to do with the fact that the image in question has a
page on commons (or at least the fact that the link is purple may be because of
this) - I have not tested it for images that don't exist at all.