Open Mail and from the top menu bar, choose Mail > Settings.
Click on the Rules tab. Click on the Add Rule button. Create a rule as follows:

Where it says Rule 2 in my example, give it a more logical name that explains what it does without having the rule. Put the unwanted domain name where I have @nogood.com .
Anything from that domain will automatically be placed in the Junk folder.
Where it says Move Message, you can change that to Delete Message. However, that is then all it does. And I don't know if that means the email will be moved to the Trash, or if it is simply discarded entirely without the chance to see what may have been in it.

There isn't an option in the rules to mark a sender as Junk. Otherwise, you'd be able to created a second action on the first image above to mark the email, and send it to the Junk folder. Though it may be automatically marked as junk just by redirecting it to that folder.