Ok, I hope I'm not on the wrong page with my rule on this one, but here goes. We had a serious issue with people showing one card, allowing all other players to fold and then mucking their hidden card without showing. It's kind of a dilemma, since they seem to be holding the only live hand, however, their hand isn't able to win the pot without both cards. So, I implemented a rule regarding the showing of only one card. I know it's a bit of a stretch, but it allowed us to gain control again.
If a player hot-dogs and shows only one card, any forward movement of the shown card kills the hidden cards. Since they no longer hold a valid hand, they can't win the hand. This leads us to what happens if they show an Ace and two other players fold. Since they were the last two players in the pot holding valid hands, and they both mucked without showing, they split the pot. Their hands cannot be pulled back out of the muck since touching the muck kills the cards. However, at the moment a player shows only one card they are holding live cards. Since they were the last to hold live cards, they split the pot.
We had to enforce this rule exactly one time. Now, every player will remind other players to turn over both cards. But, if they give them any flack, the players will call them on forward motion and kill their hand. That also happened exactly once.
We no longer have a problem (AT ALL) with players showing one card with expectations of winning a hand.