It may be beneficial for people in some space colonies to be outlawed from going to some other inhabited astronomical objects, as this could potentially prevent harmful nanotechnology and superviruses from reaching them. My main concern with this idea is that colonists wouldn't want to leave their old homes forever, even though they wouldn't be able to visit their old homes very much anyways due to the transportation costs. Alternatively, perhaps individuals going to a different astronomical object could be thoroughly searched to make sure they aren't carrying any supervirus or nanotechnology, though I don't know if this is feasible.