Paid off account but they still want more

Good day.<br> <br> I have paid standard bank full amount of what i owed them, i even have a paid up letter...<br> <br> The issue is Standard bank send a different debt collector every months to collect what is paid off, i send the guys the letter and proof of payment then they go silent. After a month a new debt collector emerges same story again and again.<br> <br> I tried to follow it up to no avail. I am sick and tired of standard bank debt collectors.<br> <br> Can someone from the relevant department clear this for good...<br> <br>