Mellins blacklist me and hand over my account but can't call me back?!

A week ago I posted this:

My first review dating back to September 2019 resulted in me returning all of the items purchased from Mellins Mall of Africa & Fourways branches. Now several months after being refunded for the items returned I receive a call from a collection agency stating that my account is in arrears and has been handed over and that I have been blacklisted... for R559.45.

How this is possible I am unsure as Mellins own accounts department calculated what was to be refunded to me and deposited it directly into my bank account, no further transactions have occurred since this refund.

If it is indeed true that Mellins have handed over my account and blacklisted me [Transunion are not showing any actions as yet] I will take all actions at my disposal including legal actions against Mellins for any damages caused and the impacting costs thereof."

Still no reply from Mellins despite replies and promises that I would be contacted. I will commence legal action on Monday.