Get a dog! =) Just kidding.
Make sure there is nothing they are eating around your house first. If there is a food supply, they arent' going anywhere.
I used mothballs around flowerbeds to keep cats from using it as a bathroom. But mothballs do smell....
I've also used rubber snakes. (which can be entertaining to watch a cat bolt away from that) But there's always one brave one that will figure out it's not real so try moving them around the garden everyday.
You may want to call pet control and find out if your area has any programs to spay and neuter feral cats.
I used to volunteer with an organization in Baton Rouge called Cat Haven (and there are others) that has a "feral cat trap and release" program.
You can ask your neighbor to contact them for help in stopping the population. If she/he doesn't want to claim them, the organization may take them to adopt out.