How to block unwanted domains?

I’ve configured Postfix to block spam using blacklists following howtos and it is working perfectly.
However, from times to times I receive lots of unsolicited mails from unknown senders. How do I tell Postfix to reject mails from those senders or domains?

 

This article might be interesting: http://www.howtoforge.com/block_spam_at_mta_level_postfix