Your bank can cancel CPA (Debit or Credit card recurring payments), and must do so at your request.
Sometimes, this is the only way to stop payments from your bank account for an unknown Now account that isn’t the one you think it is; though be prepared for Now to chase you as a result.
But then at least both you and Now will know which account is being talked about 😛
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.
If you take an offer that comes with a ‘free trial’ of Boost or another Membership, Cancel them straight away. You will still get the full duration of the trial, and if you decide you do want to continue with one or another of the trial products, just revisit the Cancel, and UnCancel it.