[Private Messaging] Limiting the users others see for messages

Hi,
We are testing using the privatemessaging system. It is a sensitive server where we do not want all members to see all other ones. We want to limit the users a member might be able to see and send messages to. Some people (internal admin) will be able to send a private message to anyone. Some users should only be able to send to approved people. Is limiting the users certain members can send messages to possible?
Thank you!
Aurelio