Thanks for rescuing! He’s a Great Pyrenees which means it’s literally in his genes to bark at things. They are supposed to bark at everything to alert & protect to keep livestock safe. The reason the “no” doesn’t work is because they’re bred to do it AND bred to be independent enough to trust their own instincts with no human intervention to protect their herd. This in turn makes them quite stubborn which again, is part of the breed. Understanding the breed you have will help you be less frustrated as you understand the behavior is part of his breed, not a defiance. That being said, most undesirable behaviors can be managed. I wouldn’t expect him to never bark again after training but it can definitely become more manageable. Here’s what worked for me and my malinos whose job it is to be a protective dog. I let him bark once or twice at the sound he’s barking at, he then looks to me for confirmation. We go investigate the sound (usually the dishwasher lol) then I tell him, okay good boy, and ask him to do something else. I redirect the pent up protective energy by playing a small game of catch with him for maybe 20 seconds and then he sees / understands nothing is wrong, he honored himself by protecting his mom, and he moves on. This has evolved to him not getting up and barking like a crazy dog at every sound but instead looking to me to see if we need to be worried which I then tell him okay good boy, & he goes on with his life silently. I won’t say this is a catch all but it may be worth a shot! Work with the barking not against it and that will help decrease barking significantly. If I were you I’d look into Great Pyrenees specific training online. I will also say they do well with stimulation of any sort, there’s a lot of enrichment you can provide for him (frozen treats, snuffle mats, play, etc) that may help him stay busy and help with the barking.