[exim] blacklisting / denying mail form exact email addresse…

Hi

I am trying to deny mail from a specific external user who continually
and maliciously spams my users. It technically is a legit email, so even
tho I pass it thru spamassassin, it never scores higher than 2 or 3.

I know this is an RTFM situation, but the rules I implemented from the
online docs seem to get ignored - and I'm running out of time :(

This is exim 4.34 and I have recently taken over the maintenance of it.
Any help would be much appreciated.

Best Regards

-gus