I need to permanently delete my account as someone is using one of my devices without my permission and using my credit card to buy membership. There should be an option to permanently delete an account or at the very least remove my card details.
Too late for you perhaps, but setting a Payment PIN would have prevented anyone buying a membership on your account.
But without that, any device to which somebody has access, and which is persistently logged into your Now account (e.g. a TV or a Stick), will allow that person to buy such memberships.
The good news though, is that this doesn’t mean, necessarily, that your credit card has been hacked.
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.