You can do this using your outlook client. Go to the properties of the public folder and set the default permissions to none. You can also set the defualt so that the folder visible option is unchecked. Then it will not even be visible to those who do not have permissions to it. Add in the users you want to have access.
This can also be done using the PFDAVAdmin tool. Download version 2.8. dotnetframework 1.1 is required and you install it on your pc, not the exchange server.