yes , you will need to make a small database to put the blocked addresses in , and reference it in your main.cf
add this to main.cf
check_sender_access hash:/etc/postfix/blocked_domains,
Make the blocked_domains db in /etc/postfix ( doesn't have to be there i just like to keep my stuff all together)
vi /etc/postfix/blocked_domains
Add the domains to it
thisdomain.com REJECT
that domain.com OK
Save the new file
Compile the database
postmap /etc/postfix/blocked_domains
Reload postfix
service postfix reload
you will need to recompile and reload every time you make a change the file that contains the domains in it