I'm not sure that immigration policy is relevant to libertarianism because a nation in some sense is like private property. So one could argue that the people of a country have a collective right to restrict who enters their borders. Transversing a nation's airspace would be a different story. I think if a nation blocked other nations from using its airspace for transversal it would be a violation of other people's rights, by abusing, essentially, private property exclusivity.