This is a a bit long but think history will make clear the issue.
I've had 2 cats for almost 10 years. They were instead of children until we had a very lucky surprise 5 yrs ago.
We also got a rescue kitten at the beginning of this year which my dd adores and calls her own.
The problem is the older female isn't normal and has always done her 'business' on the grass. Unpleasant, but duly cleared up after like you would a dog. My neighbour would occasionally complain she had done same in their garden and I could only apologise.
Now that the kitten is older it has been made apparent by my neighbour she is using their garden too (aswell as ours). When they've been away and asked us to look after their rabbits we have cleared the garden for them.
Neighbour recently has sent graphic photos whilst I've been out to a meal. She has called me round to express her disgust full force and I have done things like delivered teabags dowsed with tea tre oil to her garden to help put them off. I've stopped trying to prevent them doing it in our garden so they don't feel they have to go next door. I've apologised til I'm blue in the face but a watery bucket of excrement outside my gate this morning has me at a loss! (Rained a lot yesterday)
The cats are part of our family but I do appreciate where she is coming from. She isn't an animal person (her rabbits had matted hair and very long claws - they didn't live all that long, which she suggested was down to the presence of our cats) and had a dog once for about 4days that went back.
I don't want world war 3 and am looking for constructive suggestions of how to remedy if anyone has any??! 