See also the official documentation:
Help Center > Group Owners Manual > Banning domains from the group

Banned Domains

Users of Premium and Enterprise groups can prevent non-members from submitting registration/subscribe requests or sending messages to a group or subgroup by adding domain names to a banned list.  Adding the domain name of an existing member has no effect; they will still be subject to any group/subgroup posting restrictions that are in place.

The banned domain list is access via the Admin menu. Enter the domain to be banned and then click/tap the +Ban Domain button.  Do not include the @ symbol as part of the domain.  Use the small blue ‘x’ next to a domain name to remove it from the list.

If a non-member tries to subscribe to a group via the Web UI, a red banner will show up at the top of the page stating You are not allowed to subscribe to the group with that domain.

If a non-member tries to subscribe to a group or subgroup via email (by sending a message to groupname+subscribe@...) it will be rejected and a message will be shown in the activity log:

Non-member <NonMemberEmailAddress> attempted to subscribe with a banned domain via email

If a non-member tries to send a message to a group or subgroup from a banned domain, it will be rejected and message will be shown in the activity log:

Non-member <NonMemberEmailAddress> attempted to send message from a banned domain via email

A group Owner (or Moderator with sufficient privileges) can still use the Direct Add or Invite functions to bypass entries on the banned list on the assumption that they know what they are doing and it must be a special case.