You cannot prevent users from adding new resources but you can make a message appear. In ALL sharer files go to File > Options, Advanced. In the "General options" select All Files and uncheck "Automatically add new resources and tasks." (I suggest turning this off in any templates you are using.).

With that option unchecked, the user will get a message if they attempt to add a resource to the project who is not already in the pool file. They need to click "No." This is NOT bomb proof - a user can still select "yes" and add the resource. I would ensure your users are trained well in Project in general and particularly this area.