Uncancelable cell c contract

My Sim- only contract with them expired in May this year.June the 10th I sent them an email requesting cancelation,got automated response. End June they deducted an increased amount.Again in July I sent an email. Last month they deducted R1100.(mid month)and again at the end of the month R390.When they phone I always mention that I sent them cancelation notices but they don't bother about that,all they want is just money.How does one ever get rid of this evil.?PLEASE I WANT MY MONEY BACK