Manual:$wgAutocreatePolicy

Details
  controls account auto-creation behavior when 1>Special:MyLanguage/ExternalAuth|external authentication is enabled. (See  and  for details.)

  can be set to one of three values:


 * - do not automatically create new accounts.
 * – create new accounts on user login. This is the default setting.
 * - not currently implemented. Will require the external database to support cookies and implies ' '.