In our state the law is: if the dog is killing your livestock or poultry, you may kill it. http://www.animallaw.info/statutes/s...6_89.htm#s6552
And that's exactly what I'd do. Once a dog realizes what it can do, there's no stopping it. I would, however, for the sake of the dogs owners offer ONE second chance: keep your dogs off my property and away from my chickens or they WILL be shot. End of story. Also, in this state you are entitled to compensation for the lost animals. File a police report the next time, and try to get compensated.
It sounds like your state has similar laws for compensation, but different regarding your rights to kill the dog. Apparently you need to report it, and it needs to go before a judge it looks like. Here's the link: http://www.animallaw.info/statutes/stusncst67_4.htm#s1
Look under "§ 67-1. Liability for injury to livestock or fowls" and "§ 67-3. Sheep-killing dogs to be killed"