This post is over 3 years old, so please keep in mind that some of its content might not be relevant anymore.
When you install MediaWiki it allows new accounts creation out of the box.
This is not always the behaviour I want. Unfortunately there isn’t an option to disable it from the classic menu “my preferences”.
In fact, to disable it, a line of code needs to be added in the configuration file “LocalSettings.php”:
$wgGroupPermissions['*']['createaccount'] = false;
Done!