There is not much you can do. Hawks are federally protected, and getting a permit to kill them is a big pain in the rear end. If the chickens are smart enough to recognize the hazard, they may be safe, I mean afterall, they lived millions of years with hawks already right?
I have the same problem here and essentially after posting here and talking to many farmer friends I guess you could classify them as... was told, if the chickens are not afraid now, it's because they never seen any danger or have any 'flock mates' to yell DANGER. Also was told, once they see one of their buddies get snatched up and eaten by a hawk, they will learn and run and hide from that point on. Hopefully that does not happen, nobody wants to see that but if it does, then, it should happen only ONCE. If the hawks are nesting, they are not going anywhere anytime soon, so you'll have to deal with them.
Aaron