So angry because they still debited my account after contract cancellation

I have wrote couple of times to their retentions email for cancelling my contract . . They asked me to write an email immediately when the contract end which is the 16th Nov . . I did and now today I see a debit order taking R337.55 and my contract was R315 per month . . . I am so angry at this action because I know I have cancelled my contract with them . . Now what is the reason for them to deduct that nmoney in my account . . . So unhappy with this kind of action . . Jerrrrr