Please stop debiting my vehicle finance account

I have been calling innovation group since July 2017 to cancel the mechanical breakdown warranty and up this day, they are still debiting my account. On 20 September 2017, they sent me a letter confirmation that the policy had been cancelled effective from 14/09/2017. However, the debit order still went through and again i called them in November and i was told that the policy had been cancelled effective 02/11/2017. The debit orders have continued to this day and to make matters worse, my account was debited twice in December 2017. The first debit went through my account on 15 December 2017 with the vehicle installment on my standard bank account and another debit order went through my account on 30/12/2017. I called Innovation Group today and I was told that the policy was cancelled in November. How come my account is still being debited and was even debited twice in December. I do not want anything to do with the Innovation group anymore . Please cancel any policy that is in my name.