This cat has decided your garden is part of his territory, so he marks it to advise other cats, and this is where you start to break that cycle.
Remove all faecal matter and scrub the area thoroughly with hot water and disintectant.
Leave several moths balls in the area where the cat toilets, and place some more at the base of ytour boundary and on top of any walls or fences the cat uses to access your garden.
If - and it is a big it - a cat returns and fouls, repeat the above.
The essential aspect is to remove all traces of scent - that is what makes the mark, and what another cat will overlay, which makes the territorial marking an ongoing pee and poo fest.
That should work it's harmless, and won't hurt any cats, or your relations with your neighbour.