shoot him in the face.
obviously for legal reasons you cant do that, but keep a close eye on him and your dogs, if you catch him posining them take pictures, and take the dogs to the vet asap,
i understand his side of the story that barking dogs for no reason can become very irritating, surely there are other ways around it though, other than poising them.
Discuss the matter with him maturely, if he carrys on, harrasing you and your dogs with threats, my best advice would to be to tell the policia, im not sure they would do anything, but am sure they will if you give them a little "bonus"
Hope all works out for you, if he trys poisining your dogs i wouldnt take that kind of macho attitude, and would make sure he got poisoned too,
none of my advice is probablly legal advice, so please take it with a pinch of salt, they are just suggestions, but on the other hand people cant just go around posioning dogs when they want! especially if the dogs are under ownership,
i think the best advice i gave was my opening line, but thats my personal non legal opinion.
Hope all goes well.