I grew up on a hobby farm and dealt with the loss of numerous animals due to neighbourhood dogs. Based on my experience and info gathered to combat this problem, it is usually very difficult to "cure" a dog of its desire/instinct to kill an animal once it has gotten a taste for blood. Given the opportunity, the dog will likely kill again. I do not believe training should be viewed as futile (behavioral and obedience traing of a dog is always a good idea), but think you should consider other options to keep the chickens safe. The most obvious solution (without knowing your property set-up or if your dog is more outdoor than in, generally supervised or alone, etc) would seem to be containing your dog. Possible options could be fencing the yard, building a run, or having the dog secured by a rope/lead. My dog was an escape artist when she was younger, and I kept her tied up whenever she spent time outside. I used a very long rope so she lots of room, tied either to a tree trunk or to a metal stake in the ground (sold at most pet stores). A lead connected to a long clothesline also works. Of course if a dog is secured anywhere outside, it should be for limited times, be frequently checked on, have access to shelter and water, and away from anything it could jump/climb over (ie. a fence) and hang itself. I hope you are able to find something that works to keep both the chickens alive and your dog happy. I'm a huge dog nut and its always nice to hear of one in need being taken in to a loving home
Best of luck!