Harrased over a paid up account

My mom had a Woolworths account that was handed over to MBD which was paid up and a settlement letter was obtained from MBD which was submitted to Brook and Luyt Attorneys as the account was handed over to Brook and Luyt at the time I was in correspondence with Amelia who I forwarded the letter to and requested her to close the account but instead the account is now handed over to Norman and Bissett & Associates now and they have issued demand letters in this regard to my mother my question is because woolworths itself has confirmed that the account Is closed so where does this balance comes from that is moving from pillar to post??? And for how long will my mom will be harassed by collectors in regards to this account. This account was actually over paid and she is still not getting any feedback regarding her refund she emailed her statement to woolworths but still no response. Customer service in these companies is very poor and so is the management. I hope I get assistance from here and this matter gets resolved.