Firewall: Blocking a spammer's domain rather than the IP address?

I'm getting regular attempts to send spam from my server, Always from the same domain (yahoo.com.tw) but always from different ip addresses.

postfix/smtpd[29936]: NOQUEUE: reject: RCPT from unknown[27.41.146.243]: 554 5.7.1 <mvtwtqqj57745@yahoo.com.tw>: Relay access denied;

The server is successfully denying relay attempts but I'd really like to block connections altogether from this domain. I had just been using the router firewall but have now also turned on the server firewall, as the advanced rules allow denying access to IP address groups However, endlessly adding new IP address to that denied group seems futile.

Is there any way to create a Firewall rule that simply blocks access from the 'yahoo.com.tw' domain?