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.
@MrsT2514 Please let us know how you get on. If you are unable to achieve the most basic process of cancelling a Membership via your account then that is most concerning. Can you perhaps elaborate as to what the problem was? I know that there are many steps to go through with a string of more or less tedious "are you sure?" messages, usually followed by a special price offer, at which stage I usually succumb. But if you are not interested in that and persevere it should let you do it.