[exim] Denying mail from certain email address

Hi all

I know that Spam assassin is more meant for this.
But I would like it how and where is it possible to have some kind of
text file (list of email address) that would deny mail (send to
/dev/null) from a certain user.

One of my users is been spammed from an ex friend and this person is
send chunks of Spam.

Any advice, assistants would greatly be appreciated.

Kind Regards and thanks in advance.
Brent Clark