This is no joke. If you get tired of dealing with an annoying Comcast "customer retention specialist"[1] and simply stop paying and stop using their service, they'll keep charging you for awhile, then sell the bill to a collections agency. This hurts your credit rating, and can have all sorts of side effects and take years to repair. It sucks, but it's how things are.

[1] https://soundcloud.com/ryan-block-10/comcastic-service