I hope you've kept soem kind of record of who you spoke to, and when.
We had exactly the same problem, and resorted to shouting at the CS down the phone on numerous occasions in order to get the matter sorted out.
They were still billing at least two months, so far, after the set top box was collected.
It seems to be something to do with the disconnection department. Apparently, or at least I was told, that there was a problem with the computers around the November time, which meant that although they had disconnected my service, the computer had taken it upon itself to reconnect me !

You will agree that's very strange.
The only thing which I can advise you to do, is to stick with it, and if you have to make daily phone calls to the disconnections dept, and insist that the matter is finalised.
If you find that you can't get through, then go vis CS. I found a useful Team Leader, who did take it upon himself to ring me, when he had got through to the disconnections, and then put me through that way
I won't put a dampener on it further for you, but whilst they disconnected the account on one computer screen, it hasn't stopped the credit/invoice department from continuing to issue invoices !!!!

The matter continues, but a complaint letter has gone out, and I'll have to see.
You must keep a record of when you spoke, who to, and what was said. There is no point in expecting them to update their systems with any record of conversations.
You need to ewxplore all avenues with ntl, be able to document them, and then if needed, go to an outside body like OFTEL or Trading Standards.
Good Luck