there is a chemical that you can put down in the yard( around building and such that claims to keep snakes away. it smells like sulfur and cause the snakes minor discomfort ( when they smell the air near the stuff) lowes has it so I bet other stores do to it last like 2-5 months .
I would get some and put a trail of it around the propert edge or were you don't want them. it should either drive them out or reduce the chances of seeing them.