We had similar problems a couple of years ago, though I think they were all at it, rather there being one single offender. I tried most of the common recommendations - smelling salts, lion poo, ultrasonic emitters, weeing on the lawn (yes). in the end I think the orbit yard enforcer had the greatest effect https://www.amazon.co.uk/Orbit-62100-Enforcer-Activated-Sprinkler/dp/B009F1R0GC there are similar products on ebay/amazon for ?20-40 but they were rubbish. It's pricey but worth it. and makes for great entertainment. It only broke when I forgot to detach it during a freeze, which broke the internal valve, however since the effect has persisted. Or maybe it was something else altogether. Anyway, the upshot is the cats are pooing elsewhere and the kids can wander safely on the grass.