When I set his access to "None" he could still access the App with the deactivated User even though the deactivated User was NOT a User of the App - so there's definitely some buggy behavior. I figured the only way to restore his Admin access was to have "both" Users in the same App in the same Role; which worked.... this is one reason why I never have multiple User logins under the same email-profile; and I don't recommend it.
But QuickBase deactivated the user - but it didn't prevent me from re-adding the deactivated User to the app either.. unlike denied users.