Please, do not kill the hawk. The hawk is doing what's natural and is not like the growing population of the opportunistic raccoons or coyotes which can be pests. They are protected for a reason--they also kill rodents and other small pests and keeps them in control or your farm would be overrun with rats. We are responsible to protect our flock when we bring them onto our property. Domestic chickens are not naturally in the hawks environment and so they are really the ones that are surplus and can be killed "legally" by the hawk. Use netting, hang old CD disks and pie plates and the other suggestions already mentioned. I adore my pets and take their protection seriously without harming natural wildlife.
FYI--I would be the first to inform the local authorities if I saw anyone abusing or killing a raptor. The animal control officer who said to kill any predator did not mean raptors- probably meant coyotes, skunks, raccoons, nuisance dogs etc. If he/she did then they can be arrested and fined (possibly lose their job) if someone took them up on it and killed a raptor.