Horrible customer experience

am really disappointed with cell c once again. Since I renewed my data contract with cellc last year October they always debit extra money from my account and this time they went far by debting my account earlier while I made arrangements with them not to debit since I won't have anything in account and again they debited on the date that we agreed on. Now they send me an invoice showing that they were unable to debit but they did debited and it also shows on my bank statement. Cellc customer service is really annoying because now when I tell them that I don't want continue with the contract they tell me that I must pay R3000 for cancelation fee which they state that should I wish to cancel I must pay cancelation fee when they were selling the their product to me.