If you want to exclude specific domains from signing up onto your list. You can easily achieve this by adding such domains to your blacklist, which you can find under Settings > Anti-fraud.
Just add one domain per line and make sure you only add the domain name and extension, without
After you add a domain to the blacklist, people who try to sign up using that domain will get a "This domain is not valid." error.
NOTE: Blacklist is currently a PRO and Premium feature.