MTN incorrectly deducted an installment

I contacted MTN telephonically in October November January February March this year to cancel my contract due to high charges, problems with my phone from the beginning and poor service. the various people said that i would need to phone on the day the contract ends, which was the 15th of March, but i already notified them via email on the 8th and telephonically numerous times confirming that i would be terminating my contract. i requested on the 15th again to cancel my contract and move me to pay as you go until i could move my service provider. Now i got billed for another month starting from the 11.03.16 for a full month payment of R435.00. my contract ended on the 15th. this is *******..how can you charge a customer for a service you're not providing. i expect to be refunded immediately so that i can change my service provider to not get **********, and receive better service, as i got through to the call centre, but nobody wants to help, as they say the already invoiced me even though i have notification in march for termination before the invoice was printed. i refuse to pay for a service i am not getting. if they need to charge it should be for 11.03.16-14.03.1 (4 days