PRIVATE MESSAGING, send only to replied?

Hi there! I'm using PRIVATE MESSAGING
as a message system, and it works well and looks amazing!

one problem,..

when a user starts to type, they can see all users!
how can I prevent this?

I'd like users to be able to reply to a message if it was sent, and be able to send directly to a user if the user is pasted in send to box, but not see all users.

Or, send to only one user.
how could this be done?