League of Legends (LoL) Question: can yasuo block new nunu snowball with w !?

http://leagueoflegends.wikia.com/wiki/Nunu#PBE

According to the wiki, no.

Because it becomes a grounded projectile, which would make sense for Yasuo Wind Wall to block it(The Logic until it's verified as a bug by Riot). Doesn't make sense why it shouldn't block it while he's holding it either. The wind wall is highly concentrated air, refined & directed upward as a shield from the ground up. Why would snow penetrate that instead of getting pulled apart by the strong forces it's rolling into?

If it can stop a Speeding Bullet, it can stop a Slow-Moving Snowball.

The wind wall only stop nunu's big snowball if nunu launchs it, if he carries the ball in your dorection like sion's R the wind wall don't block it

Yes

Can confirm that Yasuo's Wind Wall blocks it. So does Trundle's Pillar of Ice, so I would assume that any champion formed terrain will block it though I haven't specifically seen its interaction with Anivia's wall, Taliyah's wall, Ornn's Q, or Jarvan IV's Cataclysm. I believe the grounded effect from Cassiopeia and Singed also interfere with it, though I'm fuzzy on the details.

For some reason, yes it can. Not only Yasuo's (W) but Heimerdinger turrets too. Both of them blocks the snowball even if you didnt recast it, even if you still holding it.
Dont know what's the reason behind this, but i've tested it and can guarantee that both blocks even if you're holding it

technically it's not a projectile as it is still connected to the champion, but i'm not sure why the logic changes when they let go of it, or if it is interrupted.