We have cats that have adopted us and live on our back deck. We feed them. There are many places to poop besides under the deck as this does create an odor for us to NOT enjoy when we are lounging.
The cats use that area because it's dark, safe, private, and comfortable. You could try covering the ground there with rocks (in addition to being uncomfortable for their paws, it will make it impossible for them to dig and cover up - however poorly - their output, which they prefer and try to do). If that doesn't work, you can close off the under-deck with chicken wire or similar. Both these methods probably will result in the cats finding another area in your yard that is hidden and has a soft layer, such as leaves, mulch, or raised bed.
Use actual lattice fencing material, this discourages digging.
Or use some Cat Scat plastic mats. The mats have flexible plastic spikes that are harmless to cats and other animals, but also discourage digging.
Or use plastic carpet runners, spiked-side up and covered lightly in soil.
Or pine cones.
I read that peppermint oil is used for repelling cats outdoors, but I do not advise you that.
Combat cat urine smell by spraying the area thoroughly with white vinegar or products using natural enzymes.
Scatter fresh lemon or orange peels under the deck. That will discourage them.
Thanks for feeding stray cats and helping them survive. As far as them using your "under deck" area as their litter box, the only way to stop them is to close it off. Staple chicken wire to the perimeter, and use those long u-shaped wires to anchor it into the ground.