Custom rules for the Susefirewall2

Hi list, I'm very new to the subject of administrating a linux server, so please forgive my dumb questions. Currently my server is receiving a lot of spam mail from a certain address. I want to block this rubbish before it hits the smtp-server, hence I'd like to add some rules to the firewall that sorts the stuff out. Is the custom script for the SuseFirewall2 (/etc/sysconfig/scripts/Susefirewall2-custom) the right place for this temporary solution? And which section to I have to use inside this script? I know that I have to setup some proper antispam-software, but in the meantime I just want to drop the mails. Any hints are really welcome Sebastian