[Exim] Blocking Specific Senders

I thought I read somewhere of a way to maintain my own list of addresses

that I could block.
We keep getting junk from free@???. How can I block this? I

wrote a filter to send it to /dev/null. Is this the most efficient
way? Or is there some global configuration setting like: block_from =
free@??? : /usr/exim/blocklist/blockfile

Any ideas?

--
Joseph Kezar