Jack,
This query appears to be the problem...
$customers_query = tep_db_query("SELECT customers_id, customers_firstname, customers_lastname FROM " . TABLE_CUSTOMERS . " where customers_firstname = '" . $names[0] . "' AND customers_lastname = '" . $names[1] . "'");
$cust = tep_db_fetch_array($customers_query);
$reason = $_POST['blacklist_reason'];
$ban = ($_POST['ban_customer'] == 'on') ? '1' : '0';
When I ran the sql in phpMyAdmin, it didn't find any records as written. I had to remove the "Where" clause to get any results. I am not sure if the rest of the contrib will work if I remove that portion of code though since there are attributes attached to the where conditions.