You could add this to /include/class.captcha.php:
$this->hash = strtoupper(substr(md5(rand(0, 9999)),rand(0, 24),$len));
$array = array("I", "J", "O", "Q", 0, 1);
$this->hash = str_replace($array, "N", $this->hash);
This will replace I, J, O, Q, 0 and 1 with the letter N.