Beware of the issue where after factory resetting a Now Stick, it will want you to buy a membership to resume using it.
I do not know how you overcome this 😢
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.