Charter Communications Kept my account open even after I turned in all equiptment and requested a disconnection. Internet

I had Charter service for a very limited time and was not bound by any contracts. I turned in all my equiptment and have a receipt of the date when it was turned in and told them I'm moving, to turn off service. The date was May 9, 2009 as stated on the receipt when all equiptment was returned, they did not turn off the service till mid september 2009 and have in their records that i did in fact turn in all equiptment on May 9, 2009. Now they are charging me from May till September for services when I wasn't even residing in the same state. I have called, written letters, submitted copy of receipt, spoke to supervisors etc... and now 2 yrs later they are still coming after me for money and have even escalated my account to collections and have reported me as having a derogatory credit to credit bureau's, which has ruined my credit score. I am still trying to clear this up 2 years later and all I get are customer service reps in Mexico City, Mexico who I can hardly even understand their broken english and refuse to discuss any details about account with me because they say it's in collection status, I have tried numerous phone calls and written letters to the collection agency whom I was told is now handling the account which is called Enhanced Recovery all to no response and no avail. I'm at my wits end and really dont know what to do at this point, I am so frustrated beyond belief that 2 years later I can't resolve this, they've ruined my credibilty, credit score and have cost me so much stress! Please help.