Only allows each user to use one discount code.

only_one_discount_code.php

<?php

/*
	Each user can only use one discount code.
	Add this to your active theme's functions.php or a custom plugin.
*/
function my_pmpro_check_discount_code($okay, $dbcode, $level_id, $code)
{
	global $wpdb, $current_user;
	$codes_used = $wpdb->get_var( "SELECT COUNT(*) FROM $wpdb->pmpro_discount_codes_uses WHERE user_id = '" . $current_user->ID . "'" );
	if ( $codes_used > 0 ) {
		return 'You have already used a discount code.';
	}
	return $okay;
}
add_filter('pmpro_check_discount_code', 'my_pmpro_check_discount_code', 10, 4);