Scenario:
We have been with the same electricity provider for coming up to six years. A while ago one of those door-to-door guys came around and offered what we thought was a better deal, so we signed up. There was a ten-day cooling off period. During the cooling-off period, we realised that their deal was only good for the first twelve months, and our original provider was actually going to continue to be better for us.
So, we backed out. We rang, cancelled with them and went back to our original provider. At the time we cancelled, they said everything was fine.
Three months later we received the first bill

We called and said "We cancelled our contract (is that what it is?) with you, and you still sent us a bill."
They said "Oh! Sorry. Yes, it says here you cancelled. OK, you're all fixed up now, you won't receive any more bills." So we received an amended bill for $0.
Then, three months later, we received another bill.
We called, said "We cancelled, and you still sent us a bill... this is the second time it's happened." They apologised again and once again sent an amended bill for $0.
Three months later it happened again, and we got cross with them on the phone. They looked into it further and said that there was some sort of glitch in the system that was causing the problem. They said they had fixed it.
It is now a few months on, and we've just received the first bill again. OMG. They won't stop. We've called them over and over to fix it and each time they say that they have, they haven't. On top of the errant bills, we're receiving promotional material and promotional phone calls that we have also tried to get them to quit.
I'm fed up completely. Who do I complain to, and how? I've never had this kind of problem before.