Cell C and their ridiculous discounts

Cell C over debited my account in April and I've called their call centre on 2 different occasions and they continue to tell me the same senseless story, I mean how can a discount expire before the end of the agreement? No where on my contract did I see that noted... you can't just go into my account and do as you please, what if I didn't have that full amount available in my account? I can't be recorded as a bad customer when you guys failed to advise me to expect a different debit order amount. I just want my money back... I have already submitted my 30 day notice to cancel my contract ending this month on the 25th... I'll never ever take out a product with you again... and by the way try to employ competent staff please, not to mention Managers and team leaders that don't want to talk to customers.