Rather than adding a new rule to the game space ("you cannot reveal more reactions against current attack after this"), you could solve this problem by using the mechanism that Horse Traders uses to solve the same kind of problem. When you reveal the card, you must set it aside, then return it to your hand at the start of your next turn.
This ensures you can only reveal the reaction once, without prohibiting two other things that might be fun to be able to do: (1) reveal multiple copies of this reaction card in response to an attack, and (2) gaining a Moat with it, then revealing the Moat.