Cancelling a debit order

I would like to cancel my my debit order.I was ******* into joining by one of your sales agents who told me she was only gaining a commission ,if I had signed up and I'll get discount when buying at picknpay.And she said I would further receive a call to cancel if I wasn't interested, which hasn't happened.Now my account is being debated with an amount I can't even afford because I'm a student.