Disconnected Service, confirmed no further payments due, still getting billed

I completed my voluntary disconnect request on 3/21 that showed that $0 additional payments/commitments were due from me and my service should have been disconnected per the form (who knows if it is, my modem is unplugged at this point). In early April, I received a bill for another month of service for AFTER the disconnect date and called the billing department that said that it should not have been billed and would be taken care of, and that I could disregard the bill. Fast forward to early May (today), I received yet ANOTHER bill in the mail for another month of service, and of course, the prior month's service and a late fee for it on the bill. 

I want a definitate resolution to this problem as quickly as possible - not one where I have to wait until the next month to see if I get another bill. Is that possible?