I had the same thing happen with Comcast. I filed a complaint with the FCC online for erroneous billing and the inability to cancel my account.

Forwarded the FCC complaint in an email to comcast and within a day got a response .A few days later they acknowledged that I was erroneously billed 600 bucks and that they actually owed me 10 bucks.

Good thing these people are now going to be our internet gatekeepers.