Are you thinking of mothballs (napthalene) and sulpher mixed? I would hesitate to use this around animals, especially chickens...

I Googled 'snake repellent recipe' and found this site that has natural cedar-based products:

http://www.cedarcide.com/PRODUCTGUIDE.asp

I'm wondering if the natural bug control recipes that mix molasses, garlic, etc. might also work as a snake repellent...?

Velvet ~:>

I used to use moth balls around the outside of the hen house where the chickens couldn't get to them cause I thougth they would try and eat them, but one got out and stayed away from them altogether. The mothballs work good for our area, also try getting a plastic egg, like what comes in a child's kitchen set, and put it in one of the nests, I have heard that once the egg is past a certain point, and the snake tries to break it inside and cannot, it also cannot regurgitate it back out.
However I have seen a chicken snake have a duck egge a good foot down it's throught and get it back out when it saw me comming with a shovel to get it. But that snake was over six feet long too.
Good luck getting it, I know it can be very frustrating.