You can kill almost anything, with almost anything. If it's a pest, and you want it gone, and aren't especially concerned with a clean kill, then shoot away with whatever you have. I would not use an air rifle unless circumstances almost demand it. If you are buying a rifle for it, get it big and powerful which, remember, is not really very powerful if we are limited to an air rifle. Then take the best placed shot you can, within the limits of shooter and rifle. I would probably choose a HP .30 caliber. A brain shot is effective, but it can be a tricky shot. A good heart/lung shot may not drop him on the spot, but it should be a quick kill, and less likely for a slight movement to cause a miss, or bad hit.