To get rid of mice in a barn you need to get some cats to live in the barn. The cats will hunt and kill the mice. You could also set some traps to catch the mice, or call a professional exterminator.
The quickest, most humane way to exterminate mice is to use an electrical device that shocks the mouse as it enters a metal area that has been baited with peanut butter or other substance that the mouse finds attractive.
