You could start with:
Control Panel » Site Permissions
Can see the shoutbox
De-Select the groups you don't want.

Then you can add a special group say user2.
That can be based on:
Control Panel » Group Management
Per number of posts.
When they reach the number of posts for user2 they are promoted auto mode to see and use it.

Blue Man Group
There is no such thing as stupid questions. Just stupid answers