Came here to answer my own question...
In qa-include/pages/register.php, I add the following code towards the start.
// limit the number of register users to 50 users.
$number_of_reg_users = qa_opt('cache_userpointscount');
if ($number_of_reg_users > 50) {
$qa_content = qa_content_prepare();
$qa_content['error'] = qa_lang_html('too many users');
return $qa_content;
}
If you see something wrong, please let me know. Thank you!