Alas, you can’t do this at present; it needs Live Chat, which is unavailable.
The best you can do is to ensure all your passes are cancelled, and, if you can, contact your bank to rescind NowTV’s CPA on your Credit or Debit Card.
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.