She is not proposing enforcing it against Russia.

Except she has said otherwise multiple times.

She was asked about shooting down Russian aircraft by Rachel Maddow. She was also asked by Judy Woodruff for a PBS interview.

Both times, she side stepped the question of would she order the shooting down of a Russian aircraft by saying the No Fly Zone doesn't exist yet and that the situation was hypothetical. Both interviewers pressed her though and she admitted that Russian aircraft have been a problem already in NATO partner airspace.

Response to Maddow asking "If say you are president and that no fly zone is in effect and a Russian jet flies into that no fly zone and they refuse warnings to leave, do you give orders to shoot down a Russian jet":

That's a hypothetical that I think there are many steps you have to go through and decisions you have to make before you even get to that. NATO did have to warn the Russians because they were invading Turkish airspace.

Response to Woodruff asking/saying "But if it came to it, it could come to shooting down Russian planes, if they’re in that no-fly zone." :

Well, let’s not talk about a hypothetical no-fly zone that doesn’t even exist. My whole idea is you put together a potential no-fly zone, something the Turks and others have been asking for for some time. You go to the Russians and you say, “Look, we want to do this because here are the objectives we are trying to achieve and we want you on board to do this.” And that’s what diplomacy is. You know, you push and you push as hard as you can. But if you just take something off the table, then you never know whether the leverage would or wouldn’t work.