I'm having similar trouble... Have a Lab/Malamute mix (bad breed to start with I know, but he came as a free puppy and I figured any dog could be trained). Problem is, I'm not the only member of the household and the other members aren't so good at the training part. I had him trained not to chase, then I left for two weeks and he started chasing. I taught him again and he was down to chasing less then once a week, then I left again and when I come back now he's chasing twice a day (or more).
He chases chickens for fun, not to kill. But he picks on one chicken and over a period of a month (or a couple weeks more recently) or so that chicken will end up dead. He's killed four. Two died from
maggots as a result of their feathers being pulled (he would catch it then hold it down and pull it's feathers), one I was told had its neck broken (I didn't see it) but I suspect actually died from complications like the rest, the fourth had a mold growing on its underside that indicated it probably died as a result of stress from the chasing. He has stopped pulling feathers (as far as I've seen), but chases more often.
I am trying the - put him on a tie, tie the dead chicken onto him and leave him for several days, while ignoring him (except to give food and
water) - method, we will see how it goes. Never done (or had to do it) before, but nothing else has worked. It's cruel, but it's getting close to the point that the only other option is to give him away (or use a shock collar) and that's kind of cruel as well.
He's a very intelligent and obedient dog and easy to train in most everything else. He clearly understands that he's not supposed to chase chase chickens, but does it anyway.
The thing is I can leave him all day in a small enclosed area with the chickens and he will not have touched one. But give him room to run (and chase) and he will chase them until they die (as long as he doesn't think anyones watching him). I guess if this doesn't work I'll get a shock collar and use the hide, watch and wait until he chases a chicken method.
As long as he's not chasing them he's actually a good guard (the number of chicken deaths per month has decreased since we've gotten him, since predators no longer get to the chickens) - and I believe he's killed 4 chickens over 7 months. So we definitely need a guard dog, and if we get rid of him then we have to go through the whole process again (though next time - if it's necessary - I'm getting a breed better suited for the job).