I read somewhere that there's something you can put on the ground that a snake will not slither across. Something about it will cut their bellies, but is harmless to animals. Let me google. I know it's not mothballs...
Ok so far I've read to plant tobacco. Snakes are highly offended by the smell of tobacco.
You can also spread dry tobacco leaves around the perimeter of the run.
Heres an idea you might want to consider. Water soluble granulated
sulfur can be spread over the yard and watered-in to keep fleas and
tick under control, but I have also read where it will keep snakes at
bay as well. Check your hardware store or farmers co-op for a supply
source. If you do decide to use granulated sulfur, its essential to
read and follow the manufacturers instructions and avoid inhaling the
dust.
And here's a link for snake repellent called Liquid Fence.
http://www.liquidfence.com/snake-repellents.html