There isn’t a way, I’m afraid.
Any mother who has tried to sit her small child in front of Peppa Pig and let it run, would heartily agree with you.
Now could implement this like the BBC iPlayer does, with a global setting for autoplay or not; this would keep everybody happy, including both the people who want autoplay, and those who like it the way it is now,
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.