Keeps billing months after account close

Account closed in August 2014 but still invoicing. Phone every month when I receive an invoice and they promise to correct the system. But Next month I receive another invoice. Now they are threatening with legal action but it is their billing system that is still billing every month on an account that was closed months ago. Phone and phone again with no success, customer service just agrees that it is a mistake and they will take it up with billing department to get another invoice next month. Sent an e-mail to get and email back saying your query has been successfully handled, just to get another invoice. No service, no interaction between departments, and impact customers financial history at their expense and they don't care.