WHM provides one more security feature for the server admin by this option. The administrator can restrict users to add a domain which is not registered. Doing this, fake accounts can not be created.
1) Login to your WHM admin panel.
2) Go to Server Configuration --> Tweak Settings.
3) You will find this option: Allow Creation of Parked/Addon Domains that are not registered.
4) Untick this option to restrict users.