Requesting a MAC does not automatically cancel the service however using it and migrating the service will/should do, basically BTW update the CP's monthly on a spreadsheet advising that a circuit is no longer theirs etc etc, this should automatically raise a cease on your billing account. As you have seen it doesnt allways do this, basically call 0800 800 150 and choose the BB Billing options advise them of the situation they can check when you migrated and enusre thay your billing account is stopped and refund any over billed charges to you. This shouldnt happen but does thats automated systems for you, but should be no problem sorting it out.
my name has been changed to save me from more company disciplinary procedures.
whatever i say is my thoughts and not those of the company i work for