Service cancellation

Plz I need help... I have cancelled my landline 1 Sept 2017. I have proof that I have send the cancellation form through to ********** Now I am getting bills which I do not want to pay As I have cancelled well in advance. Can someone out there tell me what should I do? I have tried the online chat - I have heard that the problem is with the ISP I have used. Not true - they have cancelled my service - so there is really nothing outstanding from their side. Thanks so much!