Banned Emails
From WHMCS Documentation
WHMCS allows you to ban email domains during the creation of new client accounts and, in WHMCS 8.6 and later, new users. You may want to do this if, for example, you want to prevent client signup using free email accounts.
You can access this feature at Configuration () > System Settings > Banned Emails or, prior to WHMCS 8.0, Setup > Banned Emails.
You can also ban IP addresses and configure the length of automatic IP address bans.
Banning Email Domains
The list of banned email domains displays the currently-banned email domains and the number of times a customer has attempted to sign up using them.
To ban an email domain, enter the email domain you wish to ban. For example, hotmail.com.