By default, when configuring a Domain (for example, Active Directory, LDAP, RADIUS, etc.), all the users beloging to that domain will be created automatically inside the appliance the first time they are authenticated and will be listed under Users | Local Users.
In order to disable this behavior and to only allow the authentication of users that were created before (manually by the administrator or automatically created), please follow these steps:
Navigate to Portals | Domains | Click on the Edit button for your particular domain.
Enable the checkbox Only allow users listed locally.
By doing this, only the existing users listed under Users | Local Users will be allowed to authenticate.