That would be almost impossible to stop. If I understand the question, the winner is showing one card while the pot is being pushed to him. It happens all the time. How do you stop a player from showing a card when any player at the table could request to see the complete hand if it were called, or have no right to see any card if it weren't. I know players that show one for a number of reasons. They might show a card that represents the nuts to establish that when they are in a hand they have a powerful hand. They might be using it to set-up a bluff at a later time. I don't always like it, but there are times that I really appreciate when a player shows me that I made the right move by folding. Interesting, but pretty hard to stop. This is all under the assumption that the showdown is in progress and all betting is complete.
Stuart, thanks for getting something going, and Jan thanks for your participation.
Nick C