Maybe I'm wrong, but NATO is already involved in this conflict both in a militant and political sense. Putin lied about so many things in the last two months, I think if he really wants to use nuclear weapons or whatever he will do so regardless of what we're going to do. He's gone mad, he's not rational anymore (of course I hope people around him are more sane at the moment) I'm not saying that closing the sky is the best option but we do seem to lack certain defence mechanisms from bombing.