If you still want to aim at shielded targets, but not shoot, add these conditions to "On shoot" event:
Turret on shoot
Enemy pick by unique ID = Turret.Turret.TargetUID
Enemy Touched<=0
If you don't want the turret to aim, then don't use "Add target" action, because your turret will always try to aim at these objects, no matter if they have shield or not. You will have to use "Acquire target" action for each object instance that has no shield.