As per the Catan FAQ:
Resource Cards - Do I always have to keep my Resource Cards face down?
With this in mind, consider this scenario that I've encountered occasionally when playing Catan with my friends:
Adam has just rolled a 7 and has placed the robber on a hex occupied by both Brad and Charlie. Adam has been receiving plentiful sheep throughout the game and would prefer to gain a different resource. Brad also has been receiving plentiful sheep, and currently has only sheep cards in his hand. Charlie has more diverse resources.
While Adam is deliberating about who to steal from, the clever Brad proudly reveals his entire hand to the group, showing that if he is robbed Adam will certainly gain a sheep. Adam, not wanting to gain a sheep from the robber, elects to rob Charlie, who is now infuriated at Brad for breaking the rules to sway Adam's decision and avoid getting robbed.
I believe it would be legal to say "I have only sheep", and allow that to influence the robber's choice, but revealing cards in this way is clearly a violation of the rules for personal advantage. My question is what can/should be done in the event a player breaks the rules in this way. I feel some sort of penalty is due, but what? It seems unfair to allow such play to influence the robber's choice, but it also seems unfair to force the robber to choose from the player with the more undesirable resource (this could be used strategically). So how, then, should I handle this (other than choosing players with more integrity)?
Note: This question is different from Can I show my resource cards to other players in Settlers of Catan?
My main focus is on what penalty (if any) can/should be given for this violation. Or, what incentive can be given to prevent it.