I'm with the moth ball suggestion, this way would keep the snake away from your coop but allow it to live and feed on other things like mice and/or rats depending on what type of snake it is. There's good snakes and then there's snakes that really don't help that much and can be rather aggressive too. Rats, Garter, Corns and Browns are all fairly calm snakes unless it's breeding season or they're shedding.
Depending on the size of the snake it doesn't take much of a hole to give them access. The 5 foot Rat that I posted up almost went into a 3/4" opening under my house, he got halfway in and had to stop and pull out. I'm assuming he had eaten something that wouldn't smash down enough to allow he to get through that hole.
So I'd also check out your coop and try to plug as many holes as possible too. Anything 3/4" or larger should be plugged, this will also help keep other things out too that you don't want inside your coop.
Did you see the snake? Any chance of getting a photo of it?