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.
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.