cell c unauthorised debit order after canceld my contract 5months ago

i had a contract with cell c.when my contract expired after 2years i phoned them and cancelled my contract i didnt want to upgrade.now every month theres a debit order that goes off from my account and the amount is R1200 and my contract was R280.so why do they deduct that amount? every month i have to go in to the bank and reverse the money because i canceled my contract in february and and i phoned them and told them that i cancelled my contract in february.they told me they will refund me.but yet again they deduct money yesterday.i cant go in to the bank every month and reverse the money.it cost me R20 for every reversal.and i calĺ them then i use over a R100 of airtime.i want this resolved quickly because this is *****.why do they deduct money still after i cancelled my contract ? then they phoned me to upgrade i told them i dont want to upgrade i want to cancel.so please help me to get this resolved.cell c is very bad company and im not happy with them.