function GM:ScalePlayerDamage(player,hitgroup,dmginfo)
	if not dmginfo:GetAttacker():IsPlayer() then return end
	if dmginfo:GetAttacker():GetActiveWeapon():GetClass()=="weapon_stunstick" then
		dmginfo:ScaleDamage(0)
	end
	return dmginfo
end

Timma:
function GM:ScalePlayerDamage(ent,inflictor,attacker,amount,dmginfo)
	if not inflictor:IsPlayer() then return end
	if inflictor:GetActiveWeapon():GetClass()=="weapon_stunstick" then
		dmginfo:SetDamage(0)
	end
end

hook.Add( "EntityTakeDamage", "ScalePlayerDamage", ScalePlayerDamage )