After a month of no service I called to cancel, and after 45 minutes on hold I gave up. I tried again. And again. Eventually I got through to someone who processed the cancellation. I got an email saying "To confirm your intended cancellation, send an email to cancellation-confirmation@..."
That address didn't exist. The next month they started to charge me for a service I wasn't getting. I had a different ISP by this point. I called and got the money back. They took it again the next month. I called and got the money back. The third month I got a letter saying I hadn't paid for three months, so they took three months money from my card. This time I called and got the money back and demanded they remove me from their systems. Eventually someone said OK.
Six months later I got a demand for 9 months service charges. I called and told them to stuff it. Another six months later they started demanding even more money and making threats. That's when I started the official (UK) complaints procedure. You start by writing a long email setting out your demands, end it by saying "This will also be sent by registered mail, and you have 30 days to resolve this until I report you to the communications ombudsman"
The first response I got to that was a letter saying "We understand you'd like to cancel your service?" So I sent another letter, stating my demands and that no, there had never been a service to cancel, and by the way this is being sent by registered post and you have 18 days before I contact the ombudsman.
I think by the third letter it had been resolved, almost 18 months since I first tried to cancel the account. I had to move house shortly after that though, and I've worried since then that one day they're going to turn around and say "hold on, this guy owes us for ten years of broadband! Let's ruin his credit rating!"