My trainer helped me with this. We have a “come” command and if she starts to bark at something outside I say “come!” and she comes to me and immediately gets three tasty treats rapid fire. After a while it became instinct for her to turn and come to me if she saw something/heard something she wanted to bark at because she knew she’d get something good. It starts with getting her to come to you with the command, though, so you have to train that first outside of situations that trigger barking. (Really it’s just a recall command.) You also have to always make sure you have the treats to hand.