We’re all just customers like you here, so if you want Now to be aware of your issue here, best to let them know via Live Chat:-
or by emailing customerservices@ NowTV.com (omitting the space after the @)
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.