This hook will enable/disable the user creation process. Default is allowed true means the user creation is allowed.

<?php
function my_component_init(){
		ossn_add_hook('user', 'create', 'my_component_hook');
}
function my_component_hook($hook, $type, $return, $params){
		//$params contains the instance of user 'instance'
		//You can utlize these parmaters to decided if you wanted to,
		//allow creation or not.
		 error_log(print_r($params, true));
		 return false;
}
ossn_register_callback('ossn', 'init', 'my_component_init');