I have lost three of my current flock to hawks, and one had been attacked by a hawk but survived. I knew it was a hawk (same day a hawk got another, and had clear talon marks in her back.)
What I do that has seemed to work pretty well so far considering I haven't lost any in quite a while and most of my losses were before I started actively trying to keep them away. Here are all the measures I've taken to preventing hawks from killing my chickens.
1.) I have lots of bushes, shrubs, forests, trees, and other structures (trampoline, the coop, etc.) that chickens can hide under near every wider open space in my yard.
2.) I have hung up CDs all around my yard. I read this is supposed to prevent hawks by the moving reflection confusing them. Unsure if it works, but I did it anyway.
3.) Put up a fake owl and a fake hawk on poles in the two most open areas in my yard. Try to move them once every few days so hawks will not realize they're fakes (Hawks are smarter than I'd like to think.)
4.) I have a very good rooster who warns the flock when he sees birds (or planes!) fly over, and they all scatter and hide when he gives a certain call.
5.) If I notice hawks while I'm out and about in my yard, I yell at them like a crazy person and sometimes even go lock the chickens up for a bit. Unsure if the yelling part works, do it anyway.

6.) I bought two turkeys with the intention of having them scare away hawks by their size. I read it might work. They are not fully grown, and I am unsure if it'll work even when they are. One did very intentionally chase off a crow, so I'm hoping he'd do the same to a hawk later.
7.) When a hawk does attack, I keep the chickens inside for a day or two straight. It is sad and they aren't happy, but I feel like it makes a hawk that might be returning for an easy dinner get discouraged and go somewhere else.
That's about all I've done so far. I have friends and family who have shot hawks, which is something I considered (though it is illegal, so I won't say do it, however....) but couldn't find the heart to do it, even to protect my flock.
Out of all the predators I have faced free ranging, hawks are the worst when it comes to separate incidents. The only competitor I have had with them is a dog who took five hens in one day, and other predators with chickens I've had in the past that I didn't free range. I hope some of my tips help you and your birds.