Make certain to look for any openings, of course. Snakes do not like flea and tick granules, so spreading some around areas, especially where you have found snakes, just might help in your basement. Worked for my mother's garage. Moth balls often assist in repelling animals, so it is worth a try.
Moth balls are toxic for humans and pets. One mothball can kill a dog. I NEVER recommend them for ANY reason. You need to find their point of entry and seal it. Look hard for any tiny hole in siding, foundation around doors etc. Eliminate debris such as wood or rock piles on the property.
You can purchase snake repellent or sticky traps
Cut down heavy brush or trim back hedges.
Seal small gaps under doors or decks, etc.
Use funnel type traps or glue board type traps along edges.
Seal shut any entry points into the house.
I don't know where you live but you need to find out what kind they are-Are they venomous???? if yes call pest control snake removal company. Remove their food source(mice,rats,bugs) seal all/any gaps openings inside & outside, snake repellents don't work so do not waste your money. remove any all items debris on floor and keep floors clear so you can find them to get them out.physically remove them. they are coming in to be warm or eat http://wildliferemovalusa.com/snake-basement.html