Here’s a link to my reply to someone with the exact same problem:-
Can you let us know what Live Chat replies, as it’s looking as if there might be a bug in the software here?
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.