How to deal with smug players

My take on the whole idea that, if LotR were a D&D campaign, it could have all been avoided (seen in many web-comics a few years back):

DM: Tonight we begin the campaign to destroy the One Ring, and end the threat of Sauron…

Player 1 (Gandalf): Ok, here’s the plan. Gandalf calls on the king of the eagles for a ride to Mordor. Once there, he and Frodo will fly over Mount Doom, and drop the ring in, thus ending the war before it begins. *smiles smugly*

Players 2 through 9: *smiles smugly*

DM: Alright, great plan! *rolls some dice behind the DM screen* Unfortunately, Sauron has detected the presence of the Ring because it is getting closer, and he isn’t really distracted from looking for it yet, and has realized it is rapidly approaching. Seeing this, he deduces that your plan involves flight (because how else would that even be possible?), and immediately dispatches the Nazgul on their dragons to intercept. Roll for initiative. Oh, wait, only you, Gandalf and Frodo, since your plan didn’t involve anyone else coming along.

…15 minutes later…

DM: As the fourth Nazgul falls from the sky after your unrelenting magical assault, the other five rush in for a simultaneous attack. The eagle you are riding is hit ten times (one dragon missed all three attacks), taking a total of *rolls many dice, and does some quick math* 286 points of damage.

Player 1 (Gandalf): But that puts it at -20 hit points…it’s dead!

DM: Oh. Well, I was going to give you each a Reflex save to stay on the eagle, but that seems kinda pointless now, huh? And since neither of you have the ability to fly…and I assume Gandalf doesn’t have feather fall memorized?

Player 1 (Gandalf): …no.

DM: Right. So, you both plummet to the ground below, taking *rolls many dice, and does some more quick math* 125 hit points each.

Player 2 (Frodo): *looking dismayed* Frodo is dead…

Player 1 (Gandalf): So is Gandalf…

DM: Ouch. With a sickening crunch you both land amidst the orc army gathered around Orthanc. *rolls some dice* The good news is that you managed to kill seven orcs with your impact. Yay! Seven blows for the alliance of men and elves!

Players 1 through 9: …

DM: The bad news is, the Ring is found, and immediately taken to the Tower, where Sauron uses it to fully come to power, thus completing his quest for the domination of Middle-Earth.

Players 1 through 9: …

DM: *putting his stuff away* Great session, guys! So, who’s up for starting the supers campaign next week?