What to do with neighbour's cats that are running rampant?

My mother in law is having a problem with her neighbours - they're, well to put it nicely, 'trash' who have 7 cats that roam outside at all hours. That on its own isn't a big deal if they kept to themselves, however these cats constantly trash my mother in law's garden and she is regularly finding cat shit all over her vegetables.

Given their attitude (when she asked them to keep their cats in their yard, they started hurling racial slurs at her), there's no hope of them taking care of the cats themselves.

Anyone know of a humane way to keep cats away that is foodsafe and doesn't result in a bunch of cat corpses?