Blocking Spam

I'm having issues with receiving spam messages from a particular email address.....

Using FF, and ATT Mail client, under Options / Spam / I have listed us.mc1817.mail.yahoo.com as to be blocked. However, we continue to get ~ 50 bogus email msgs from this site daily!!

Even switched to another email svc, and eventually, they found us again! Any thoughts on how to block this???