We don't have a big snake problem here, but I've heard people put glass eggs in the nest boxes, and when the snake eats them...well, that's the end of the snake I guess.
I've only come across about 3-4 snakes and only 1 was in a nest box. (they all showed up within about 2 weeks of eachother) I relocated each of them to an area about 1/2 mile from us, and never saw any more since then. That was 2 years ago. No doubt I will be covered in snakes tomorrow for saying that.
I got 5 guineas about 4-5 months ago. They're suppose to keep snakes at bay, eat fire ants and ticks too. Although...as noisy and nasty as they can be, sometimes I think I'd rather deal with the snakes, fire ants and ticks. Other people love guineas...I don't mind them, when they're not around. lol
Good luck! Oh, and if you have copperheads, I've read black snakes will keep them away, so I'd rather have the black snakes than the copperheads which is why I did the relocation thing.