Sprint Continually Billing

I closed my Sprint account in June of 1999. Shortly thereafter I paid the bill in full. I received another bill for late charges only and another bill a month after that. I wrote on the invoice that the account was closed and paid in full and sent another check for the balance due. I received another bill with late charges only again. I sent the bill back with a note that it had been paid. I received another bill with late charges added to the late charges. I sent a letter to their customer service with a payment history and explaining everything. I received another bill with late charges added to the late charges. When you call them you get a recording that they are so busy that you will be put on hold for at least 15 minutes before you can speak to anyone. I haven't been able to reach anyone no matter how long I hold. Every month they are charging me a late fee (this month $.25) and a service charge of $4.95 plus $.58 taxes. How can I have a service charge and taxes on an account that has been closed over 7 months.