Not sure who is being unreasonable here but here we go.
Two neighbours. I am neither neighbour.
Neighbour 1.
Has a cat. It's always in neighbours 2 garden. It is doing its business in neighbour 2's garden. This has been seen several times and the cat gets chased out of the garden by neighbour 2 all the time.
Neighbour 2 is getting a bit fed up and doesn't want to put up spikes because it will deter other wildlife which they love.
Neighbour 1 will not cat proof their garden. It's been proposed by Neighbours on either side but they won't entertain the idea.
Neighbour 2 has been dropping the cat poo back over the fence into neighbour 1's garden. Neighbour 1 is just leaving it in a little poo pile where it's been dropped.
Both neighbour 1 and 2 are pissed off with each other.
Do cat repellents work? Will they deter other wildlife? I mean the sonar sound ones.