Nothing turns a game of Hearts around like a player who successfully shoots the moon. Shooting the moon, or taking all of the points (penalty cards) in the game, harkens back to the origins of the game of Hearts. Hearts, like Bridge and Spades, is a derivative of the classic game of Whist, but Hearts evolved in such a way that the goal is to avoid taking tricks and points (whereas the goal in the others is to take them). Successfully shooting the moon in a game of Hearts will award the player negative 26 points, or, conversely, 26 points added to the score of every other player. While shooting the moon can be of great benefit to you, your game can be completely destroyed when another player gets away with it, so it is definitely in your best interest to do everything you can to keep it from happening. Depending on the play style of your opponent, this could be a fairly simple task or an incredibly onerous one; fortunately, there are some tips that you can remember that will help you keep your opponents from shooting the moon.
Watch for the signs
First of all, watch for the tell-tale signs that someone is planning to shoot the moon. Player who lead with high cards (such as aces and kings), especially in hearts, are very likely trying to take points, and the only reason that someone would actively try to take points is if they plan to take all of them. Likewise, during the passing phase, consider the cards that you have been given. If someone has passed you three low-value cards (such as the 2D, 4C, and 7S), there is a good chance that they're trying to get rid of their weakest cards so that they have a better chance of shooting the moon. Be careful not to confuse this with a player simply trying to create a void in one suit, which typically means that they are finding a way not to take any penalty cards.
Hold some midrange cards
Hold on to one or two cards of mid-range value, such as a ten or jack of hearts. Cards like these are easy enough to slough off once you see that the points in the game are being distributed among the other players, and they can be invaluable when someone attempts to shoot. Players often shoot without having perfect hands to do so, and therefore their success is reliant on their strategy. If you can hold on to a middle value heart long enough to take a trick with it, then the points that you take can destroy your opponent's plans, and the few points that you take with the card will seem minimal, compared to the 26 that you would have taken if you hadn't intervened.
Watch your score
When attempting to stop a moon, however, you should consider your own score. If you've played well enough to have a comfortable lead, taking a few points in order to stop the moon will pose no problem. If taking points and preventing a player from shooting the moon would push your score over 100 and cause the end of the game, however, you may want to hold off. As with all other aspects of the game of Hearts, timing is everything.