Normally you wouldn’t want to stop your Maremma from barking.
Barking is how they communicate and warn off predators.
He may believe their is a threat present and warning the predator off.
But if you do want to stop him from barking you could immediately call “Thank you” after the first bark, then a pat occasionally for staying quiet.
Remember to also reward quiet occasionally before he barks so he doesn’t get in the habit of barking for no reason.
With some practice, he may bark just once and then look at you for your response if you are close by.