Struggling to cancel landline service

I have recently upgraded from ADSL to fibre. As a result, I no longer need my landline, as the only reason I had one was for the ADSL requirement. The ADSL add-ons have all been cancelled. I followed the instructions to cancel, but received no response. I then followed the cancellation process again, but no response. However, the bills keep coming and I continue having to pay for a service which I am not using. Please could someone assist me with the cancellation of this service.