Keep on bugging me about account that is paid in full

MBD keeps on calling me and sending me messages about an account that is paid full. I've told them the account is paid up, and even faxed through all the slips to show them I've paid the account, but still I'm getting calls every day from them. I don't even answer the calls anymore. I don't know what to tell them for them to stop calling me. I cant keep on faxing slips, it seems they just throw the papers away.