Manual:$wgAutocreatePolicy

Details
controls account auto-creation behavior when external authentication is enabled (see $wgExternalAuthType and $wgExternalAuthConfig 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 ' '.