If you live in an area where weasels are native, killing them isn't going to do you much good. Build a weasel proof pen and keep your chickens in it during the weasels' active and chickens' inactive hours. That way your chickens are safe and you get the varmit eating advantage of the weasels. As long as you keep up on the maintenance of your pen, its the only permanent solution that completely stops the loss of chickens to predators and also keeps the presence of rodents down to a manageable level. Hardware cloth, either 1/2" or certainly 1/4" works well, keep in mind that poultry wire is only to contain chickens, it isn't intended to keep predators out. The best/easiest way to go is to put the hardware cloth over your existing wire and also either use it as an apron, going out 2' from the exterior wire (and connected to the upright wire) or down 18", slanted away from the perimeter. I use the apron method, lightly covering it with soil. I have used poultry wire for the top of the pen and so far anyway, it has worked well.
