Moderation will be part of the next release - that should help you with that and achieve exactly what you want! In the mean time, you could hook yourself to the event thrown when question are created and set them as hidden - only admin will then see them.
Another option for you would be to create your own theme and when outputing question list, you could ignore all those that are unswered for not logged in users.
Finally, built in right now and with no coding involved, you can only go to Permissions and decide to show your questions (all of them) only to registered users.