@Mail has a feature called "Deny Senders". The Denied Senders are domains / addresses from which you will never receive email. You can enter mailhosts, country codes and email addresses as addresses not allowed to send to your domain.
You can enter Regular Expressions or exact word-match expressions as a filter for your "Deny Senders" list. More info on Regular Expressions can be found here. You can also specify which users/groups are covered by the blacklist, or choose to implement the blacklist globally.