Block Access to Distribution List

I am running Exchange 5.5 on NT 4 with 2 sites A and B. I want to block the users that are on my Site B to be able to see the Distribution Lists in my Site A. They should only be able to send e-mail to the receipients in Site B. Currently they are able to see all the receipients when they compose a new message..
Is it possible and if yes How ?
I am running 2 servers with a x400 connector between them.
Thanks