Misleading clients

When I upgraded my contract, I was switched to a top up contract. They explained to me that I when I reached my monthly limit for data / calls, I would need to call in and request a top-up. In Aug and Sep 2017, my data was depleted and I called in to request additional data. Now in Oct 2017, I am automatically charged for the top-up. The call centre explained that it will happen every month unless I request to stop it. I asked them to cancel for Oct, but they say it's already been charged ad cannot be reversed. They call it "Available Spend". My Contract is R129 pm, but my bill for Oct is already R396 for additional data / airtime I haven't even used or requested.