Capitec playing judge and jury with my accounts

I have a main account with capitec and then a secondary linked account. I put money into the secondary account as I have had a really tough month and main account had a negative balance with outstanding debit orders.

Well over night Capitec decided to take money from my linked account to my main account to offset the negative balance without any kind of authorisation from me they just did it and informed me its done!

Then when I reached out to them I get told well its irreversible as you main account cannot be in a negative.

I am clutching at straws because Its been such a tough month and this is honestly just pathetic service.