Charter Communications - digital Cable phone internet Charter Bills post disconnection Madison, Wisconsin

Charter Communications is a nightmare when it comes to billing. I was a charter customer in Madison, WI from Aug 07 - Oct 08. I moved out of WI in Oct and had asked Charter for disconnection of services during the 1st week of Oct which was verbally confirmed by their rep. I was declined a confirmation number/email from Charter about disconnecting services despite asking. I had not noticed until June 09 that I was actually billed (money debited from my account) until Mar 09 from Charter in uneven amounts (ranging from 30 - 120) to the tune of 500$ after Oct 08!! I called charter and the rep confirmed of incorrect billing and said he'd escalated the issue and I'd be getting back the money within 6-8 weeks which did not happen. Every call after that I have gotten different answers and finally someone said I would not be given any money since the equipment was not returned. I had to travel back to Madison and return the equipment! I got a check for 99$ for the equipment but nothing about the extra  500$ that has been wrongfully taken. I called the customer service again who asked me to send the bank statements for the credit department's review (he said I should fax it to the local office @ Madison) which I did; Finally when I called them up to chase the matter, I am being told now that Charter only keeps financial records of current year and since I am not a customer anymore they cannot see in their systems as to what I am talking about!! WTF??!! I am being now told that *nothing* can be done about it! I am at a loss here... the same guys (everyone uses the same system out there, I'm presuming) who told me they'd chase the matter up are now saying to my face nothing can be done. I am willing to sue the company, but not sure what options I got...