Well, the problem with using punishment is that it does not result in any *predictable* change in behavior. That is the primary drawback to any punishment as a training tool. Even if you punished him every single time he ever saw a chicken, it is no guarantee that the behavior would stop. And honestly, if you are punishing a dog that much on a daily basis... what kind of life is that for a dog? Even if the method of punishment is fairly benign, nobody would enjoy being nagged daily their whole lives.
The best solution in this type of situation is good Management. Keep the dog away from the chickens and the chickens away from the dog. You could use crates, fencing, tie outs, etc... and of course each would have pros and cons. Failing that, you can either rehome the dog or rehome the chicks.