if ( isset( $_SERVER['PHP_AUTH_USER'] ) ) {
  $banned_users = file( '/www/path/website/ban.dat' );
  if ( is_array($banned_users) ) {
    foreach ($banned_users as $user) {
      if ( $user == $_SERVER['PHP_AUTH_USER'] ) {
         header( "Location: http://www.domain_name.com/ban.htm?" . $_SERVER['PHP_AUTH_USER'] );
      }
  }
}