Having taken out a contract with a firm for one year on line and payed by credit card how can I ensure that the firm do not renew without my authority and charge my credit card in the future?
It depends what sort of agreement you signed up to, when you paid by credit card. If it's a continuous direct debit, rather than a one-off, they can continue to take it - you did give authority in the first place. Ask the firm for a copy of what you signed (if you didn't keep a copy at the time), so you can check.
I did not sign anything. It was all done over the phone and on line. They did ask if I would like a two or three year agreement but I declined to see how the deal worked for one year. It just worries me that they have the card no.
Write to both your credit card provider and the firm supplying the contract you do not want the direct debit to be taken from your account after whatever date you want it to close.