Any "block" feature cannot be based on the email address. It must be based on the sender's computer IP address and you can bet spammers have ways around this also just as they constantly change the bogus sending email address for their garbage.
Are you doing anything to contribute to the problem such as automatically rendering all HTML received?
If a spam message is correctly marked as junk by the Mail.app Junk Mail filter, no HTML will be rendered even if you automatically render all HTML received. But if a spam message includes embedded images or objects that must be rendered from a remote server to be viewed and these are rendered, you are revealing to the spammer that your email address is valid or "known good" causing even more spam to be received.
Have you ever clicked on any links included in a spam message? If so, the same applies.
Do you use your .Mac email address when making an online purchase or for any website access if required? If so, you are opening yourself up to receive more spam.
Once the cat if out of the bag, it is too late since spammers sell or trade valid or "known good" email addresses to other spammers.
I haven't experienced a major spam problem with my .Mac account but I do get a number of spam messages sent to my POP email account provided by my ISP. I forward spam messages (with long headers shown for the received message) to my ISP and if my ISP determines a forwarded message is spam and they can obtain a valid IP address for the sender from the long message headers, my ISP blocks that IP address. Sometimes my ISP reports no valid IP address is found in the long message headers and in that situation, there is nothing they can do.
Check
Why was I "spammed" at my .Mac Mail address? and
How .Mac filters spam.
The .Mac mail servers filter spam, but some unwanted messages may still get through.
No single strategy is completely effective when it comes to managing Unsolicited Bulk Email (UBE), also known as "spam". People who are intent on sending spam are ingenious at coming up with ways around any filtration schemes.
I have no experience with the Symantec spam program since I don't use and avoid using any Symantec products with OS X. The Mail.app Junk Mail filter has always worked very well for me but I also avoid doing anything myself that contributes to receiving more spam.
