I tried to mark an email message as spam in my Earthlink email. I got the following reply:

The option to add a domain to your Blocked Sender List is not available with this selection. If you selected multiple messages when you clicked This is spam, try selecting one message at a time to reveal that option. Note: The option may still not appear if a message’s headers contain multiple domains.

I only selected one email to remove, so I assume that the note at the end applies.