J.C.
Wow, you must have bought our old house! Congrats ;)
First of all, everyone we tried to contact about getting the cats out of there (er, helping the cats) said that because they A) were outside and B) had no tags or registration, they were 'feral' cats, though we all knew who they belonged to and she too kept food and things for them quite obviously on her front porch. It was horrible - we even tried to contact someone for her health as she was quite elderly and her entire house reeked of cat urine and there was feces on the carpeting everywhere inside her home. But no can do, as she was determined competant and able to live as she pleased.
So....
After much trial and error about what kept cats away from the yard, the neighbors who had lived there longer then we had told us to use moth balls in our yard. Worked amazingly well. Since we have little guys and pets, we just bought a couple of boxes, which have the moth balls inside a plastic bag, and threw them around the cat's favorite areas rather then taking them out of the bag. They lasted forever, we probably did that a year before we moved and never had to replace them.
Best of luck!