@adaline
The problem with cats comes because they're have the right to roam and in reality owners have no control over them once they're outside of the house.
Unfortunately, roaming cats can be a nuisance - they shit in people's gardens, get in their houses, terrorise their own cats, spray and can cause accidents when they run out into the road too.
Mine are outdoor cats and always have been, (in 20 years of having cats,) and they have certainly never behaved like you describe.
Also, 10 of the 30 homes in our cul-de-sac have cats, and no-one else's cat behaves like this either.
They may have plopped the odd turd out on someone else's garden, but I doubt it's very often because our 3 lawns (front and back) have cat shit on them, and so do our lawn borders (and so do our planters.)
And I have caught our younger one (age 10) shitting in our planters and on the lawn(s) many times, and the older one does it in our lawn borders, so they probably don't do it in anyone else garden.
Cats will only generally behave like you describe, if they are not cared for/looked after properly, and have owners who are out 12 hours a day, and leave them alone. (So they are wandering about looking for a family that cares and will let them in to their home.) And yep, dominating the existing cats is a symptom of this issue.
The OP, and most other cats owners are caring pet owners who have well-behaved cats. No way in HELL is putting spikes on a fence like the OP's neighbour did, acceptable. Don't care whether it's illegal or legal It's fucking nasty. Hope the woman is charged with animal cruelty.