Please close my account and stop billing me

I had a contract with mtn which has come to an end.I was told to give in one month notice if l don't want to upgrade but to close the account which l did give the one month notice on the 3rd of March this was the reference number l was given when l made the call ***.This week l receive a email which states that l will be billed for April which l do not understand because l have closed the account why am l still be billed and l have followed their instructions to say l must given in one month notice for my account to be closed.MTN I HAVE FINISHED PAYING FOR MY CONTRACT PLEASE STOP BILLING AND I HAVE FOLLOWED THE CORRECT PROCEDURE TO HAVE THE ACCOUNT CLOSED WHY AM L STILL GOING TO BE BILLED IN APRILL I JUST WANT THIS ACCOUNT CLOSED PLEASE.Account number BA***