i cancelled the contract but money was debited

I have a contract with MTN and the contract ended on the 20 April 2015. I went to MNT Alexandra on the 15 of may to cancel the contract and they it is too late they will debit on the 20th as it takes 5 working days to cancel. i understood and told me write a letter for reason for cancelation and i did so and they also took a copy of my Id. I was surprised that they debited again this month on the 20th as they promised it will be cancelled. i want to know why my money was debited as they even get airtime and data. i want my money back as i cancelled the contract.