I want to terminate my contract with IMMEDIATE EFFECT!!

Terminating a Contract with car track is such a HASSLE. I’ve been trying to cancel my contract since April 2024, the agent that assisted me told me if have to pay a cancellation fee, I told him I’ll pay the contract till my 36th month which is November 2024. I paid everything on the 25th of November and I have nothing in Arreas and sent an email for cancellation and I asked them to cancel with Immediate effect. But I keep getting calls from the accounts department saying that my account is in arrears. I informed the agent that I paid everything, sent a proof of payment and a cancellation form to client services. I am now being informed that my cancellation will be effective from the 27 of December, and they will still debit me for the month on December.