BasilMod::SafeZone is your choice!
This is server-side only mod, no client modification required.
Define zones free of combat, where your citizens will be safe. Newbie Zone, NPC city or GM Trade Post.
Mod does not punish rule breakers (i.e. players who still tries to attack, even if it is not possible), no damage return or insta-death applied. yet. Only warning is shown.
This mod is part of BasilMods bundle.
Video demonstration
Server-side installation and configuration