At the absolute very most the guard's blow should be 1 point of non-lethal damage and an attack roll should be made. You should also be very clear in your description that the guard is attacking non-lethally; something along the lines of "amazed at your impertinence the guard admonishes your rudeness by striking you in the face, do you wish me to roll for attack or do you let the guard strike you for 1HP of non-lethal damage?"
This. Because the attack by the guard is within the non-combat part of the narrative, I believe the player should be given the opportunity to determine whether they want to keep to the non-combat part of the narrative, or "take it to the flip-mat" and move into the combat part of the story.
Another idea is to not have the physical blow do any damage, but instead be part of the narrative description of an Intimidate check by the guard, resulting in a Shaken condition for the character.
In my mind at least, any guard/watchman would have Intimidate in their social skill set.