Immediate cancellation

I have done everything in my power to cancel my contract with O2, I finally managed to get a confirmation of cancellation on December 29, 2025. However today, I called regarding a bill received for January and the person on the phone stated that my contract is still not cancelled. What else do I have to do so O2 cancels this contract? It's been over a year that I've been paying for a service that I'm not using. I demand a confirmation that the landline internet is cancelled and that I won't be charged anymore for this service. If I don't receive a confirmation from O2 by Friday I will take legal actions for this abusive treatment. 

Please cancel the contract immediately and stop charging me for a service that I am not being provided.