Haven’t used Now TV for a while and I now have new free trials available for cinema and entertainment (and kids & hayu though I never used them). Shame it’s only 7 days nowadays.
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.