wizbones has some good points but you have to think about what happens if they dont want to come back.  Now you have users on the server that arent going to be used anymore and are taking up space.  Two weeks isnt a good time line though.  it should be more like 2-3 months before deletion or thought of deletion should occur.  I think that if no posts are made in 2 months that on of the moderators or the admins should email that user to see if they still want to be a member or if they dont mind being deleted.