There is a small button on the side of the Stick.
While this is powered on, hold this button in for 15 seconds while the Stick reboots.
See if this fixes it.
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.