you could change the recipient restrictions in the postfix config to and add this:
The sender_checks file could look like this:
# This file must be "compiled" with "postmap"
# Using a domain name
example.tld 554 Spam not tolerated here
discardmail.com 554 Spam not tolerated here
izmail.serveftp.net 554 Spam not tolerated here
# Maybe example2.tld is on a DNSbl, but we want to let their
# email in anyway.
# We get lots of spam from example3.tld, but we have somebody
# there from which we do want to hear
and when you have the file the you need to "convert" it do a db --> postmap sender_checks