Hi johari,
In my experience, a dog that's killed more than once is very, very committed to the sport.
The best thing would be (while still training it not to kill) to always, always keep them separated by a sturdy fence.
If you want to give the dog the best chance at learning it would mean keeping the dog on a leash to start with, and reinforcing that it shouldn't even look at the chickens. Take it around on your daily chores, always on a leash, always correcting it when it tries to look at the chickens (or needless to say chase them). I don't believe positive reinforcement alone can do the trick; I believe you need to correct the behaviour with a firm but not painful tug on the leash. (People will differ about this but I've found that firmness is much much better understood by the dog than trying to win it over with treats.) The ideal is a combination: treats for good behaviour and correction for the slightest bad.
Gradually the idea is to start doing the rounds (once the dog has learned to stop looking at the chickens when you tell it not to) with the dog off the leash but still under your control. Sometimes it's good to encourage the chickens to act flighty so you can test the dog's training so far; if the dog goes for the flighty birds then training has to start over, with a firm leash tug and so forth.
Always make sure you reward good behaviour, not necessarily with food, but with warm praise and pats.
I have a 6 month old pup who is very chase oriented, so I'm doing all these things with her, and I'm finding (2 months into the process) that she's coming around. She doesn't look at the chickens any more unless they really start to squawk and squabble, and if I call her through the chicken flock she tiptoes around them, as she knows she's not allowed to upset them. However I've seen her still eyeing them off hungrily when I'm not out there with her, so I have a feeling she's never going to be 100% reliable with free ranging birds. In this situation a fence is going to be my best bet.
You'll never get a not-very-well-socialised or disrespectful dog to behave around chickens. The best chicken dogs are those that absolutely understand their place in the family and really want to help. A dog that sneaks off on its own is more likely to be wilful and prone to attacking what it sees as sport or prey.
I reckon 2% of dogs are ideal, 15% okay if trained very very well, and the rest are going to be best kept behind fencing wire at all times (while some will be unreliable even with the best fences).
Just my thoughts, all totally personal and non-expert of course.
cheers
Erica