You can find this section in HostBill Admin → Settings → Security Settings → Banned Emails.
This feature allows you prevent to ban traffic from certain domains/email addresses, to help you get rid of spammers that overload your support dashboard.
To add email domain to banned list simply enter the domain, optionally add the ban reason and choose if/when the ban should expire.
Users that will try to register using email accounts from listed domains will get error message about rejected email, with reason you provided when adding that restriction.
Each entry also provides a counter that shows how many times this domain was rejected.