Absolutely disgusted with their service

On the 1st September they deducted almost R2400.00 from my account, (monthly subscription is R874.00). I contacted customer service and was refunded R770.00. According to them I owed R496.00 (no one can tell me why) and considering I have paid by debit order for the last 16 odd years I really cannot understand how I owe them money? (In sixteen years I have never received an invoice or statement). However you do the maths this doesn't add up and despite having contacted customer service telephonically on numerous occasions as well as numerous emails nobody has had the decency to come back to me and no further refunds have been forthcoming.