Go to Subscribers and click on Manage Subscribers. Search for the addresses in that domain by entering the domain in the field next to Search for Email Address. Click the button to search. When the results are displayed, click on Toggle All to select all the email addresses. Then click Deactivate Checked to unsubscribe all the ticked email addresses. The email addresses will remain in the system, but they will be Inactive, or unsubscribed. You may reactivate them later by performing the same search, ticking all the records, and clicking Reactive Checked.