Family Mobile Walmart Trying to double charge customers Kansas City Missouri

When you sign up for Family Mobile, you pay for the month in advance, and each consecutive month after that is paid for in advance.  When I called to cancel, I was informed I was going to be billed for two more months.  I told them I refused to pay for two more months.  I cancelled one day before the due date and every month was paid for in advance and I will not pay for two more months on a service that is month to month with no contract.  I also informed him that if I do receive the bill, I will be contacting BBB and a lawyer since it is a prepaid service and not billed after use.  The reason it is called "prepaid".