I know that cats (and their owners) are above/beyond the law when it comes to leaving crap in my flower beds - it's annoying/vile but that's the way it is.
But ...
... (you saw a 'but' coming, didn't you?) - what about if the cat is being walked on a lead?
I've just seen a young woman taking a cat for a walk on an extending lead - and they paused by my front garden whilst the cat disappeared into the foliage and then emerged with an "I've just had a dump" air about it.
I didn't challenge her because I wasn't 100% what the cat was up to - by the time I had visual and odoriferous proof they were well gone up the road.
But could I have insisted she clear up (she had no bags with her as far as I could see)?
strange isn't it? any dog faeces left in the country lane where i live is met with fury but the mountains of horse manure in the road almost cause a celebration
You could have run after her with it, put it in her hand, and said you were concerned that her cat had dropped it and you didn't want her to lose it. No thanks necessary, just being a good neighbour.
Not all cats hide their mess for you to find unexpectedly. (Although the one that messes on my kitchen extension flat roof does.) In my experience as soon as they have found a favoured flowerbed, lawn spot, vegetable or flower pot, then they leave it proud, to show what they think of you.
I'd like to think if my cats had donned collars and leads, and joined the dog in her excursions,I'd pick up after them. Particularly in a neighbouring garden. It would seem cheeky not to.
I'd like to think that if I was daft enough to try and put ASBO on a lead I would clean up after him. Although might be difficult with all those bandages.
I found hot curry powder, mixed with hot water, and put in an old window spray container does the trick, to keep them away, but try to avoid their eyes!