How do I block an email address in Postfix?

How do I block an email address in Postfix?

I need to block an external email address in postfix from sending me
emails. This is an external email address of a third party domain name
that I'm not controlling.
The reason why I need to block it is because they have something
misconfigured and I'm getting a message saying "Warning, your message has
not being delivered yet", every second or so. I already contacted their
tech support, but they are taking a long time to fix it and in the
meantime, my server and my users are suffering.
I tried doing this. In my mail.cf I added:
smtpd_sender_restrictions = check_sender_access
hash:/etc/postfix/sender_access, permit
and in /etc/postfix/sender_access I added:
mailfilter@iomart.com REJECT
I run
postmap hash:sender_access
and restart postfix, but it seemed to have no effect.