cell c keeps debiting my account

its been 2 months since I settled my cell c contracts phone but each month they keep debiting my account the very same amount i used to pay them off.i have called them 4 times now,they always promise me it was sortted and I wont have the same problem,again but it keeps happening.i am very angry because i dont owe them anything and it is always my duty to go fix all the damages they cause when they debit my account.today they debited about R6000,last month about the same amount again.im dissapointed n very angry.i will never again buy a phone from cell c and I will make sure most people i know dont ever use this nertwork again.