installment

i have an account with barnetts . on the contract i noticed that there was a deduction of a magazine , which i feel i dont need it . i called and requested for same to be cancelled and it was , however they are still deducting the same installment of R614.00 . Each and every month i receive a mini statement via sms and it states that my installment is R589.96 however thy debit the R614.00 . can the installment be adjusted accordingly and i must be credited all the money they took for the magazine since the time of cancellation .