Firstly cancel your direct debit at your bank, if you haven't done so yet, so they can't deduct anymore money from your account. Here is a letter you might send, inlude exact dates, exact amounts and send it to the right address. Please note i this is just a draft, i am not a proffesional writer so if anyone wishes to correct it feel free to do so.
Dear Sir
R.E. Termination of Internet Service
I cancelled my internet service with you a while back, however you still seem to be deducting monthly fees from my account. The date of my cancellation was (_DATE_). Please stop debiting my account. You now owe me (£_AMOUNT_) in respect of the overpaid monthly fees for (_HOW_MANY_) months. I have already telephoned you on (many occasions) (_DATE(s)_) requesting the refund of which I was informed I would be refunded. As of yet I have not recieved any money or correspondance regarding the situation and you still are deducting monthly fees. I would like to make it clear, I have cancelled the service so please stop taking money from my account, and refund the debt you owe me. I look forward to hearing you. My contact number is as follows: (_CONTACT NUMBER_). If I do not recieve a reply I shall be forced into taking legal action.
Yours faithfully,
Sign and print name