Several years ago I saw a TV show about this guy who had loads of black widow spiders on his property. In my opinion, he devised a great way to kill them. First he went out and bought a cheap can of hair spray. He would go out at night when they are most active and hunt them using a flashlight. He would then give them a shot of hair spray. When the hair spray dries, it becomes stiff and the spider can no longer move, thus it dies!
Rich