Nedbank charging interest on zero balance bond

On 22 December 2018, I paid my bond in full. Thus, the outstanding amount on the bond is R0. Yet, every month, Nedbank charges an admin fee and interest on an admin fee. When I called the call centre on 5 March 2019, the operator informed me that the bond cannot go in dormancy state and that it is absolutely normal to charge an admin fee and to charge interest on top of the admin fee. This is not true, I have had this bond in dormancy status before and during dormancy, no admin charge is levied. In addition, how can interest be charged on an admin fee when the admin fee in itself is not outstanding? These actions by Nedbank is in contravention of the National Credit Act and the Consumer Protection Act