Unauthorized premium deduction

I discontinued my son from the medical aid and the medical aid advised me via email that the last premium will be deducted from my bank account on 01/03/2021. I have not made provision for yet another deduction and they still went ahead and deducted another premium on 01/04/2021. I find this totally unacceptable and need this premium to be refunded to my bank account a.s.a.p.