Some enemies have combat restrictions:

If an enemy has a “no swords/axes/spears”symbol on it, players are not permitted to assign that type of Viking Warrior to fight that enemy. Viking Warriors of any type may travel on a longship to face an enemy but Warriors of the prohibited type are destroyed by the enemy before the first round of combat. They do not participate and are discarded to the general supply without ever being rolled.

It is permitted that a player assigns no dice to a combat location.