You can turn it off, which will save you having to give it every time you watch something for which the PIN is not mandatory, such as individual movies, no matter what their rating.
But in the circumstances in which it is mandated by Ofcom (live channels which have not undertaken to show post-watershed material only after the watershed) you still have to give it.
i.e. the Off in On/Off saves you having to give the PIN in many places, even though it it is still overridden in some places.
But as above, don’t blame NowTV for having to follow Ofcom’s broadcasting rules. Their stance is that no matter how innocuous a programme is when the channel is switched on, and no matter how child-free your home, it may be that a child will arrive later, and innocently settle down to watch your TV, now showing Saw 3.....
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.