MBD Paid up but still confusion on their end

Have been having an Issue for weeks where i have received a paid up letter confirming that the account has been settled and closed, however, i am receiving SMS's saying i owe money. Been calling MBD and have even sent the letter back to them, but it seems that there is no central system. The service has been extremely poor in resolving this matter. Seeing that i have written confirmation, this is considered harrasment. I am still waiting for feedback after numerous calls. The consultants cannot allow me to escalate the matter as there is no process, which i find hard to believe.