No-fly zone at the moment is an automatic declaration of war on the Russian Federation. There has been legislature in Russia which controls deployment of their forces to Syria. You cannot just pack your stuff and go back home if U.S. planes appear on the sky. The point of the very deployment to Syrian soil is to render these 'proposals' from U.S. hawks ineffective. If Russia supported SAA from outside, e.g. from international waters and long range aviation from Ru airbases, then Russians could stop once U.S. is flying over Syrian skies.
Besides, if U.S. tried to impose no-fly now, Russians wouldn't ICBM American mainland, they would use conventional weapons to take care of U.S. equipment in the theatre. Kirov battlecruisers can wreck anything several hundred miles in circle. Since Assad is still the legitimate leader of Syria and he has made an official call to help against terrorism, U.S. would be an agressor and Russians just need to respond in kind. They don't need to fire the first nuclear shot. It's 2016, they have enough conventional weaponry to symmetrically respond. Unless U.S. brings 2-3 carrier battle groups and a shit ton of planes in allied bases around Syria. But that kind of escalation has several steps and lasts a while. It's not happening