BEING BILLING BUT I HAVE CLOSED THE CONTRACT

I TOOK A WIFI ROUTER ON CONTRACT IN NOVEMBER 2016 AND CELL C KEPT BILLING ME MORE EVEN THOUGH I HAVE ASKED FOR A CLOSED LINE IN APRIL 2017 I CALLED THEM TO GET MY BALANCE IN ORDER FOR ME TO CLOSE THE CONTRACT I PAYED THE MONEY IN FULL, THEY KEPT GETTING INTO MY ACCOUNT TAKING MONEY I CALLED AGAIN AND SENT THE PROOF OF PAYMENT AND I WENT TO THE STORE AS WELL FOR THEM TO SEND THE PROOF AF PAYMENT FOR ME AS WELL BUT I KEEP BEING HARASSED BY THEM CALLING ME AND TELLING ME I HAVE I BALANCE DUE, CAN THIS GET SORTED