WHY MUST I PAY FOR A CANCELLED SIM??

WHY HAVE YOU DEBITED ME FOR THIS MONTH WHEN I REQUESTED A CANCELLATION LAST MONTH??? ITS NOT AS IF I CAN EVEN STILL USE THE SERVICE AS IT HAS BEEN SWITCHED OFF AND AS FAR AS I KNOW WE PAY IN ADVANCE SO WHY THE HELL WOULD YOU STILL DEBIT ME THIS MONTH? PLEASE REFUND MY MONEY , I WONT PAY FOR SOMETHIING I DONT HAVE.