Termination of membership

I have written a letter requesting termination of membership to South African Democratic Teachers Union. They asked me to provide me with the necessary details, which I did and they even acknowledge receipt of the information they needed. I have continued to write countless e-mails to request them just to terminate my membership but to this day since April 2015 they still deduct my money from my persal because they dont want let me go out to another teachers union. So I am already with the other union but they still continue to ***** my money unauthorized. I want them to return my money and terminate my membership with immediate effect.