Unfortunately I am not likely to be able to help you. I get large numbers of emails from people troubled with garter snakes in their basements and gardens, especially in northern states in spring and autumn when they congregate around hibernation time. I do not know how to prevent such intrusions, and the only things I can suggest are to make sure any possible entrance points into property are sealed, as snakes can get in through the tiniest of holes, and to make sure that gardens are as open as possible, with little clutter around, as snakes don't like open spaces, and do like secure hiding places.
Also, it is important to remember that garter snakes are entirely harmless, and pose no threat to pets or young children. Indeed children are not naturally afraid of snakes, and need to be 'taught' phobic behaviour by their parents or grandparents.
Reptiles have been on this planet an awful lot longer than human beings, and we are visitors in their world, rather than the other way around!
In the long term the best solution for both man and snakes is to learn to live together, and an acceptance of this situation, along with seeking treatment if the phobia of snakes is severe, are strongly recommended.
If your enquiry is not about how to get rid of unwanted garter snakes, then please go ahead..........!