Complaint re unfair charges

I continue to be billed for services I have not accessed but for one month only (Dec 29th - Jan 29th), due to being informed by Carphone Warehouse that all I needed to do was cancel my direct debit to stop the service. The service being used was a wi-fi dongle paid monthly like a pay as you go service as I was advised. Although ID is able to look into my account and see I have not accessed it since the date given above, I have been receiving letters and emails saying I owe £30.00 per month for Feb, Mar and now Apr. ‘Missed/late’ payments have also been registered against my name on my credit file, to my dismay! The reason I signed up in the first place is because I was told I could cancel the service after one month if I chose to, just by cancelling the direct debit. Although, I was also advised, I would have to go through the process of re-registering. I therefore cancelled my direct debit after one month, only to be billed constantly since. I am absolutely appalled thus do intend to take this much further at the earliest opportunity. Absolutely ridiculous!