cancelled service, but still getting billed

I moved and cancelled service Jul 20, and got a confirmation email.  I paid the month Jul 7 - Sept 6 and was due a partial refund.  However I keep getting billed for more months.  Aug 6 and 31 I called and got cancellation again, postdated to Jul 20, but still getting more bills and no refund.  Who do I send details to, to get my account billing fixed?