Maremma is a breed with low prey drive. Good news is that you can successfully wean the bad habit. You have to train them. You have to first teach them that its not an acceptable behavior. You can train them by enforcing that you would not accept aggressiveness towards chicken. Be very strict, don't hesitate to use a stick to threaten them. It's a negative enforcement, but sadly I can't see how you would be able to convince them to never mess with a chicken again otherwise.
Next time, when you are introducing a flock guardian puppy have a Mediterranean type rooster, the type that gets people aggressive. Usually they would teach the puppies to keep a respectful distance. But, in your case, you have elapsed the window of opportunity.
Dogs are pack animals, you have to assert yourself as the pack leader. I emphasis on being very, very strict. Dogs are wonderful companions and Maremma usually have good lifespan. If you teach them what is acceptable, you'd have a wonderful guardian dog, otherwise you'd raise a predator.
I don't beat my dogs, but I did teach them what is acceptable. My female lab killed a pigeon when we got her. We got her as 1 year old. I kept the carcass next to her and made her understand that I really don't accept that behavior. Now 2 years have gone and she doesn't even pay them any attention. I don't condone physical abuse on dogs, but for your dog's own good, it needs a lesson.