MTN contract

Im so disapointed about MTN service. I called them in December requesting them to cancel my contract because of poor MTN network in my area. They asked me to pay cancellation fee which i didn't have at that time or wait unti 2nd February because is due for renewal. I waited and sent the email on the 2rd of February to cancel it, they rep**** requesting my personal information for verification and i sent them.

Today i received a bill invoice. I thought they already cancelled the contract, now i must pay for the data which I'm not using.

How can i stop them to debit from my account?