$this->ModelName->read(NULL, $id);

unset($user['User']['modified']);

$this->User->save($user);

public function beforeSave($options = array())  {
    parent::beforeSave();
    if (isset($this->data[$this->alias]['modified'])) {
        unset($this->data[$this->alias]['modified']);
    }
    if (isset($this->data[$this->alias]['created'])) {
        unset($this->data[$this->alias]['created']);
    }

}