If you cannot get the issue straightened out, you may need to file suit in small claims court or county court for return of the overcharges. That would at least get the company's attention and require them to provide an official explanation. Having received the letter and acknowledged the same without telling you that there was a signature issue, the company is likely going to have a difficult time convincing a court that they didn't know that you were cancelling your service.
You can reach Harkess & Salter LLC at (303) 531-5380 or info@Harkess-Salter.com. Stephen Harkess is an attorney licensed in the state and federal courts of Colorado. This answer is for general information only and does not create an attorney client relationship between Stephen Harkess or Harkess & Salter LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.
I would suggest that you dispute the charges with your credit card company. I assume that the "Vivint" charges are being auto-charged to your credit card, contact the credit card company and dispute any charges and cancel the auto charge. Secondly I would write letter, certified mail, to "Vivint" including copies of whatever you have previously sent them telling them to stop charging/cancel the contract, refund your money, etc. Security companies can be very difficult about allowing consumers to cancel, and they frequently auto-renew if you do not cancel. Hopefully you have proof of what you faxed to them back in May of 2011. You might also contact your state Attorney General or Consumer Protection office to see if they can offer any assistance. Best of luck!
This is not legal advice and no attorney-client relationship is formed. You should retain an attorney to receive legal advice on your particular situation.