At one point, I accidently told the iOS app to save my security key when it prompted me after signing in, How can I turn that off. I want the app to prompt me to read my security key (via NFC) each time I log in. I have lost a way of keeping my account secure and it is one of the reasons I got Bitwarden.
You need to login to the web vault, go to Settings->My Account, scroll to the bottom and click “Deauthorize Sessions”. This will log you out on every device and reset any “Remember me” 2FAs that you already have set, so you will need to log back in (with 2FA) everywhere.
After that, as long as you don’t check the “Remember me” box, and you log out (not lock) then it will always ask you for your security key when logging in again. There is a setting in options to set “Vault Timeout Action” to “Log Out”, which will cause your vault to be logged out instead of locked whenever you Vault Timeout happens.