If you want the training to work it must not come from you. The dog has to make a decision that he wants nothing to do with chickens. In other words if you tell the dog no, you just made a decision for him. You have not convinced him chickens are the devil.
So put a collar on him and leave. Go inside the house, grab a chair, a drink and a sandwich. When the dog goes for the chicken wait until he just gets ahold of it and let the lightning go. Hold that button down until he turns to run away, if he looks back, hit him again. You let off pressure when he is headed out. So with you out of the picture the dog is forced to make a decision and that decision is chickens are the devil.
My little black bitch used to go coop and get the chickens. I sat there for 2 hours and finally I saw her run by the window. She went into the coop and I couldn't see her. I gave it 5 seconds and switched the mega watts on. When she came out of the coop she was tumbling and howling. That has been 5 years and I still can't get her near a chicken. She is the tpe of dog that likes to stay at heel ALL THE TIME. When I walk towards the coop she stops. If I call her she puts her head down and takes one step. I call her again and she will take another. It is obvious she thinks the devil lives in the chicken coop.