Some administrators may wish to prevent abuse of their server by blocking registration domains from specific countries or of specific origins (e.g. free domains). cPanel offers this as a feature.
User-denied domains are controlled via the /var/cpanel/commondomains file.
In the Domains Section of Tweak Settings (WHM > Home > Server Configuration > Tweak Settings), you must enable "Prevent cPanel users from creating specific domains."
You can add full domains or just top-level domains to the file.
# nano /var/cpanel/commondomains