How can I atone for the sin of giving poisonous medicine to my neighbor's poultry, and what is the way to deal with their continuous harassment and attempts to sabotage my project?

You must repent and seek forgiveness from Allah, and compensate the injured party by guaranteeing the value of the damage inflicted upon the chickens. You must also deal with the abusive neighbor with patience and repel evil with good. If this is not possible, or if the neighbor persists in their harm, and mediation or reporting to authorities does not help, then Allah Almighty answers the supplication of the distressed. One should weigh the harm currently experienced against the potential harm of moving.