If the cost to Baron to publish the letters is X, and the blackmail payment is Y, precommit to reject any offers with probability greater than 1-(x/y). That shifts Baron’s expected value to negative (unless x=0), and increases the expected value for Countess by less than (1-(x/y))*(z-y), where z is the loss of value to Countess of having the letters published.
That strategy deters every Baron who is sophisticated enough to be deterred by a full precommittment, and does better against Barons who proceed regardless.
If the cost to Baron to publish the letters is X, and the blackmail payment is Y, precommit to reject any offers with probability greater than 1-(x/y). That shifts Baron’s expected value to negative (unless x=0), and increases the expected value for Countess by less than (1-(x/y))*(z-y), where z is the loss of value to Countess of having the letters published.
That strategy deters every Baron who is sophisticated enough to be deterred by a full precommittment, and does better against Barons who proceed regardless.