There are two ways to deal with this, as mentioned: In game and Out of game.
Out of game, a simple talk would suffice -- here is what metagaming is, here is why I don't want to see it in my game, and if I do see it in the game, it tells me that you're cheating, and therefore, you're going to be kicked out. As a DM, you should be able to have that private conversation right there in the room with the other players, and the other players should have their heads out of their back pockets enough to avoid using any of their in-character knowledge. In the same vein, your players should be able to make a Spot, Sense Motive, or Search check, roll a 1, and say, "Okay, no problem there, full speed ahead!" instead of saying, "Well, I don't see anything, but maybe I should check again, just to be sure."
In game, my solution would be simple. Observe:
DM: Hey, P1, come out into the kitchen with me for a private conversation. (goes into kitchen, knowing that group is eavesdropping) The elf says, "When you get to the temple, you'll see a giant red ruby. The person who touches it will gain permanent mightiness and become so physically powerful that rocks will bounce off his chest like sand."
P2: Bwaha, I'm eavesdropping! I'm sneaky! Ya-hoil!
DM: Hey, P1, good conversation. (Comes back into room.) Okay, you all head out and show up at the temple. It appears to have been the site of a major battle.
P2: Is there anything interesting, um, like a giant ruby?
DM: Why, yes.
P1: I head for --
P2: I run over and grab the ruby!
DM: Okay, as you touch it, a beam of red light flares out. The rest of you are momentarily blinded, and when you can all see again, P2's PC, and all of his items, have been turned into stone.
P2: Hey! That's not what you told P1 would happen!
P1: Dude, no kidding. Good thing I didn't touch that thing.
P2: Suck!
DM: A moment later, an anvil falls from the ceiling and crushes the statue of P2's character. You'll need a resurrection spell to bring him back now.
P2: Oh, man! This is so--
DM: What is that? A Gust of Wind trap? D'oh, it looks like most of P2's PC's body has blown away. I guess it's gonna be a TRUE resurrection or Wish now. Man, too bad you ran over and grabbed that ruby like that. Hey, P3, your bard sees something written over where the ruby was resting. Your Decipher Script check reveals that it says, "Cursed Ruby: Do Not Touch".
P1: Ah, if only he'd stopped and waited for us to search for traps.
P2: But you told him what it would be!!!
And at that point, if P2 hasn't figured out that you are going to use each and every one of his little cheating experiences to utterly shaft him, you should give up and start pelting him with four-sided dice until he leaves.