I can only suggest that you arrange a meeting with the cat itself and tell it how you feel. Your neighbour is powerless to change your cat's behaviour, that is why she has a CAT and not a DOG.
Your neighbour's cat will shrug at you and shit in your garden more often. Best to think of a cat other than your own as you would a fox, badger, hedgehog - although if you are the sort of person who's inclined to shoot these things, that would not be acceptable - and take measures to protect your property from poopattack. This can be done in many ways.
Think of a cat like you would a burglar. Asking a Burglar's mum to 'have a word' is of no use. Reasoning will get you nowhere fast. To deal with a burglar you plant prickly things of fences, you sort out your home security, you turn your home into a veritable castle. With a cat, you plant things they don't like (ask your local garden centre, I have a cat, so I plant catnip!) or plant things they DO like (catnip) in an area you don't mind them being, you spray your garden with (whatever its called) 'stuff' to make them not wish to poo there (again garden centre sells it, we use it on our bins... we have foxes, badgers, hedgehogs AND 'other' cats... it works a treat).
There is stuff you can do to limit the poop, you can even rid yourself of the problem without ridding your neighbour of their cat! Speaking to your neighbour, or the cat is POINTLESS. Fish need water, Cats need to roam (I don't do housecats, it actually upsets me a little but I accept not everyone can let their cat out as much as they need, and the cat is usually happy - ish) so this is a problem for YOU to solve, not your neighbour, not your neighbour's cat.
It's a PITA though, I feel your pain, and YANBU to utterly detest the whole thing, especially if your're not a cat lover!
Cats learn very quickly to avoid things they don't like. And you have every right to powerhose use a water pistol as well... for your own self gratification if nothing else, but after you've soaked it a few times, get to a garden centre and ask advice!