<?php
/*
  Disable Registration based on Register Helper field value
	Add this code to your active theme's functions.php or
	a custom plugin.
*/
function my_pmpro_registration_checks($okay)
{
	//only check if things are okay so far
	if($okay)
	{
	//	global $pmpro_level;

		if(isset($_REQUEST['not_permitted']) && $_REQUEST['not_permitted'] == 'yes')
		{
			pmpro_setMessage("You are not permitted to register for this level", "pmpro_error");

			$okay = false;
		}
	}

	return $okay;
}
add_action("pmpro_registration_checks", "my_pmpro_registration_checks");