2 Answers

http://www.insidefacebook.com/2009/12/10/now-facebook-lets-users-hide-friends-from-people-who-are-not-logged-in/

No. Either you go public or you go private, but you can restrict the people who can see your wall, (but not if they are within your system). I.e. you want some people not to see you or find you in a search then you block those people specifically either by their name or by their known email address. But you cannot block the whole world nor would you know the aliases they may use nor the emails that they may also use which are total fake. You can also limit the answers on your wall to "friends of Friends" or "Friends only". Once within your system, they can see what is on your wall, but they cannot write on your wall, unless they are friends. You can also use the "comment" feature to limit those who can comment on your wall by setting it to "me only". "Unfriend"/"unsubscribe" is the best option if you uncertain about people.