Unauthorised set off app**** to my account

I am totally disappointed with the service i received from Capitec. I received a msg saying that one of my accounts was in a negative balance and they have app**** a set off- meaning they have transferred money from my account with a positive balance to the one with a negative balance without my permission or even letting me know that this is it. Their terms and conditions do not even have a clause regarding "set off" . I am totally out of pocket now. I was not even aware that my account was in negative as on the app, it just showed a balance of R 0. 00. Yet over R 2000 was app**** as a set off. Surely they are not allowed to do this without my consent. I had even called the call centre and was told " Sorry, There is nothing they can do to help me" I will be taking this up further if my money is not transferred back into my account asap.