Currys PC World

I bought a television from Currys, was asked by the salesman if I wanted product support, I said several times that I did not but he was very pushy and didn't want to take no for an answer. I was replacing the television because we had been burgled the day before, and had been up half the night with the children, who were quite upset, and the last thing I wanted was to stand there arguing. Eventually he said that the first month was free and he could cancel it straight away, so I agreed for him to do that. I've since realised to my annoyance that It was not cancelled and I am not able to cancel the direct debit because of the way the payment has been set up. How do I get this payment stopped?