Punishment for acting as you describe should be having their morale annihilated and charisma set to 3. Adventurers and heroes shouldn't be snivelling babies.

Further, it should be considered bad luck or a curse to call someone a coward in Thera. Set up an automatic detection for tells with the word "coward" in them, and have every use zap some of that mythical luck stat away or after a couple of uses have the speaker become affected by an uncurable curse for 200 ticks.