on damage of player:
if attacker is a player:
if victim isn't wearing armor:
cancel event
send "&c&lHey!&7 You can't kill unarmored players." to attacker
if attacker isn't wearing armor:
if victim is wearing armor:
cancel event
send "&c&lHey!&7 You can't kill players while not wearing armor." to attacker
It's just 9 lines, this is how easy it is!