If you have taken out a trial directly with Now, just cancel it, all four steps, within the seven days (and best to cancel before the sixth) and you will not have to pay anything.
But if this isn’t what you have done, please tell us what it is you have done.
PS: I’m beginning to see why BT try to ‘lock’ the Now apps of their subscribers to their BT TV Now account numbers 😛
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.