It is easy to use your default WordPress admin interface to block comments from a certain domain. You can either have these comments require moderation or you can have the comments immediately marked as spam. My suggestion would be to initially have the comments require moderation just to make sure you are not also catching legit comments.
To block comments based on a certain domain follow the steps below.
- Open WordPress Admin: First visit your sites WordPress admin located at yourdomain.com/wp-admin and login.
- Open Discussion Settings: Expand the Settings menu in the left navigation and select Discussion.
- Add to Comment Moderation: Now add the domain in question to the “Comment Moderation” window. If you want to add multiple domains simply add one domain per line in the Comment Moderation box. Please note that WordPress will catch anything containing the phrase you add. Say the word “mail” has been added then any comment containing the word “email” will also be flagged for moderation. You need to be careful you are not blocking comments you want posted which is why I suggest testing the domain in “Comment Moderation” before moving it to “Comment Blacklist”.
- Save Discussion Settings: After you have entered the domain or domains into the “Comment Moderation” box simply scroll to the bottom of the Discussion settings window and click the “Save Changes” button located at the bottom left hand side.
Once you have tested the domain entered is only blocking comments you want blocked you can move the domain into the “Comment Blacklist” window so you are not required to view each comment that has been detected to include the domain in question.
The WordPress site has a great article, located here, discussing other methods to fight comment spam.