Hopefully they have now agreed to cancel the service without insisting on 30 days notice. You should cancel the payment mandate.
It is up to you whether you want to try to recover payments made. This will involve standing up to the provider and your bank. If you do want to recover money, you need to be aware of the direct debit guarantee. This states that, every month when there is a change in the payment amount, the organisation receiving the payments is required to notify you of the date and amount before they take the payment. If they did not do this, then you are entitled to ask for the payment to be reversed. It is possible that the bank will refuse to reverse old payments but they certainly should reverse the last few amounts.
You should write to the provider (don't do it by phone) stating that you cancelled the service in 2010 and you have not used the service since then. You should ask for a refund. They will refuse the refund and they will ask for payments for any amounts that get reversed by your bank. It is possible that they will involve debt collectors and/or legal action so you should keep a copy of your letter.
To get payments beyond those that get reversed by your bank, you will need to consider legal action (or a counter claim to any action against you) and/or involving the banking ombudsman.
It is important that you are certain in both your own mind and in what you say in communications that the service was cancelled in 2010. The provider will take advantage of any doubts that you may have or that you may infer in correspondence.