In section Configuration. Note: Sysops can still create accounts directly. To disable this, add to LocalSettings.php:
$wgGroupPermissions['sysop']['createaccount'] = false;
This causes an error message on 1.23.2 "Notice: DB transaction callbacks still pending (from User::invalidateCache)."
Instead $wgGroupPermissions['*']['createaccount'] = false; Still allowing only sysops to create accounts works fine.