NPC Proficiencies (DMG)

As a convenience for the DM, non-player characters are assumed to be proficient with the weapons they carry. However, this need not always be the case. If you want to make an NPC easier to defeat or less dangerous, rule that he is not proficient with his weapon. This is most likely the case with simple innkeepers or townsmen impressed into the militia.
The innkeeper may be adept with a club (occasionally useful in his trade), but the niceties of swordplay are not within the normal realm of his business. By adding to or subtracting from the abilities of an NPC, the game can be balanced and enriched.