Admin.banUsers

(official documentation on admin.banUsers)

PHP example

require './facebook.php';
 
$facebook = new Facebook(array(
  'appId'  => 'YOUR APP ID',
  'secret' => 'YOUR API SECRET',
  'cookie' => true, // enable optional cookie support
));
 
try {
   $result = $facebook->api(array(
        'access_token' => $facebook->getAppId().'|'.$facebook->getApiSecret(), // use app signed token
        'method' => 'admin.banUsers',
        'uids' => array('4')  // ban that pesky Mark Zuckerberg
      ));
} catch (FacebookApiException $e) {
  error_log($e);
}

The method returns a boolean value as to whether the banning was successful. Note: it returns true even if the person was already banned or the UID doesn't actually map to a legitimate user.