Once a chicken snake had found your coop, nothing will keep him away, if you have used small enough wire and no holes anywhere, you should be fine, but you still have to meet the chicken snake on your porch on the back of your chair when you are not expecting it....lol
Bolded for emphasis. For folks with stand-alone chicken coops, it just takes a little work to keep the snakes (and: surprise-- rats and mice too!) out.
I used to live in Georgia and TX and have had experiences with 'whoa, lots of snakes' including some of the more aggressive species such as cottonmouths. I actually had someone try to hand me one, out of ignorance, once... but that's another story! I'd still welcome them around, for they are the best rodent control one could ask for, and they work for free. I respect that folks would want to remove or kill a venomous snake where they have children or pets, I really do. Protecting your children and animals is important. But the 'any kind of snake, it's dead' attitude is sad, especially since most snake predation of chickens/livestock can be avoided.