If this came out of the blue without you having had any prior contact with Now ever, then someone has your card details, and you need to get your bank to stop the card ASAP, and give you a new card as soon as possible.
However, I’m guessing that these perhaps aren’t quite the circumstances; so what are the exact circumstances?
Set a Payment PIN on your account so that no-one but you can buy memberships on it. Check your bank accounts monthly for any other unexpected payments to Now. That way you can at least nip them in the bud, while you and Now figure out whose fault they are.