Which version of Moodle are you using? I don't know how to make it permenent but this works in my Moodle 1.9xx and Moodle 2xx as long as I stay logged in. Add a filter that "is empty" the first time you go to 'browse list of users.' Of course if you log out, it will have to be done again the next time you log in. I am sure that you could probably dig through the code and make that filter setting your default.