How can I keep cats from using my front lawn as their bathroom?

You can get a motion detector hose attachment to spray anything that walks near the sensor.... cats hate it.... https://www.amazon.com/Hoont-Powerful-Repeller-Activated-Squirrels/dp/B01N3P1JTG/ref=sr_1_fkmr2_3?ie=UTF8&qid=1511144014&sr=8-3-fkmr2&keywords=motion+detector+hose+attachment+pet+deterrent

You've used a lot of the items suggested for keeping cats out. There are a few more in the article below. Scroll down to find the right section. Best of luck.

https://www.alleycat.org/resources/how-to-live-with-cats-in-your-neighborhood/?gclid=CjwKCAiA9MTQBRAREiwAzmytwwXttDsmt6znibUoZHL_rOCtCwnW5ttm4jRcLmoiCUzU9SQyL66jBRoC-6UQAvD_BwE

mint works great for animals and foul to stay off lawn., also cucumbers in area for ants

You could spread out some shredded Irish Spring bar soap. You can shred the soap bars on a box grater or even in a food processor. Animals hate the smell and taste. Wash down you rocks, chips, etc. before to get rid of most of their scent.

Moth balls will keep out cats, snakes, and squirrels. I once had a cat spraying on my flowers and killing them. I put mothballs in a plastic bag and placed in the flowered. No more yellowing plants.

You could get a motion detector sprinkler. It comes on when animals enter the zone, and will startle them away.