Contract still not cancelled

After almost 4 months since I've cancelled my contract and logged several calls for the issue to be resolved, my account is still not closed and I'm still being billed. Why is it impossible to cancel my account? I have spoken to several different people, I need a manager or senior team leader to phone me because the call centre agents are clearly not doing their jobs. I have to keep on paying rejected debit order fees because you keep on trying to bill me for a cancelled contract.