Because the "Reply To" fields in email headers is very easily forged, and because spammers will rotate IP addresses so that blocking a single IP doesn't work either, I've not had much success blocking domains. However, after realizing that most of the garbage I was receiving was coming via servers in South America, it occurred to me that in my circumstances, if ANYTHING passed through those servers then I did not want it. Since it's very hard to hack the Received field values, I tried the following:
As you can see, it gave me the ability to filter on subsections of IP addresses. This seems to have worked fairly well at the time, although it doesn't seem to be getting used as much now since my ISP seems to have finally gotten around to blocking those things. It might be worth a try.
(BTW, I'm not the Jeff_W apple community specialist that was posting here earlier :-)