How Do You Stop Someone From Shooting the Moon in Hearts?

Short Answer
A player can only shoot the moon if they take all 26 penalty points in the hand. To stop them, make sure they don't take everything. The simplest defense is to take at least one heart (or the Queen of Spades) yourself.

Signs Someone Might Be Shooting

The Simplest Defense: "Break" the Moon Shot

If you can win a trick that contains a heart, do it. The moment anyone other than the shooter takes even one penalty point, the moon shot is dead.

Example

You suspect Player B is shooting. When hearts are played, you spend a high card to win a trick containing a single heart. Even if you take 1 point, it prevents +26 to you and the other two players.

How to Force Them Into a Loss Trick

Common Mistake: Helping the Shooter

The biggest moon-shot mistake is playing "safe" the same way you always do. When a moon is on the table, "good score" strategy changes.