Impossible to cancel a month to month contract!

I have sent the form to cancel my data contract *** on 7 September and they advised me on the phone that it takes 20 days notice, so I was still in time to cancel for end of September. However, they have still not cancelled and they have now debited my account twice from the date of cancellation. I cannot afford this contract and have advised them as such. I am now in debt because of this and I still havent had any feedback! I cannot keep paying for this service!!!!! I also need to be refunded for the past two payments. I have followed up numerous times and opened a case: ***1 with no avail.