Delete all received emails except incoming emails from a particular account

Hi guys.
I have directadmin v.1.51.3 whit installed smapassassin.
I want drop all incoming emails to a specific account except all emails received from another specific account.
in the other words, i need the A account only receive email from B and other emails drop.
I try write some rule on exim.conf or exim.spamassassin.conf file but they don't work :-/
How can i do that?