CAPITEC BANK WON'T REFUND FUNDS I ACCIDENTALLY PAID TO THEM

I'd like to express my absolute disgust with the Service i received from Capitec and Nimble Collections,Which collects on their behalf.I mistakenly made extra payments on a FULLY PAID UP loan and on requesting a refund I'm being sent from pillar to post by the two,both saying funds are not with them.How is that possible? This are transaction that are backed up by bank statements yet none of them wants to account for the funds.I'm beyond sad over how they conduct business and hope i NEVER have to deal with any of them in future.