, if the snakes are "always near the back-porch", then there's either good cover to retreat to, or a sunbathing / basking spot.
I'm going to bet there's a crawl-space under the porch floor - a nice shady hidden area that's out of the sun, but not too cold, & safe from big predators [like dogs] that would make a meal of a snake.
I'd wire the porch frame from the underside framing to the ground, with an in-ground installed hardware cloth
, & make sure that it's galvanized - to slow any rust down. Bury at least 12-inches vertically, & another 12-inches horizontally, running OUT from the porch-edge. That way it won't easily be dug-under by burrowing animals, such as groundhogs, large rodents, or even rats - just in case there are any, locally.
[Snakes will use larger animals' burrows as an access point to the porch, if any critter dug under in order to reach the crawl-space.]
U can use S/S staples & a stapling handgun to secure the wire. // Hardware cloth is not cheap - it's welded
at every intersection, so it's much stronger than other woven-fences; 1/2-inch openings are good, they keep out the PREY of snakes - which would also tempt them to follow after.
I'd also eliminate low-growing brush near the house, & any larger stones - basking surfaces are very valuable to snakes; i'd take them out, b/c in order to thermo-regulate, snakes move into & out of sun or shade.
Trees are fine, I'd put landscape-bark or another non-grass cover atop a layer of WEED BARRIER, so water can enter the ground, but grass won't sprout; this also protects the tree trunk from being bashed by a mower, or sliced by a weed-whacker. The cambium that feeds a tree's canopy is thin & fragile, & 5-minutes with a weed-whacker circling a good-sized specimen can girdle & kill it.
Brushy or thick-growing stuff, such as decorative clumping grasses, are perfect cover for snakes - open branching shrubs that allow U to see the ground & the interior of the bush are much safer.
I wish there was a commercial odorant of BLACKSNAKE or KING SNAKE, as both of them will eat other snakes, & venomous snakes avoid encountering them - laying down a boundary of their scent as a spray would greatly discourage incursions.