Look into closing in the carport. That's probably the best way to fix it. Plus I would imagine it would increase the value of your house.
If that is to expensive of a solution I would look into getting a no kill squirel trap. You can catch a cat with those. I've caught cats and rabbits with mine. They don't hurt the animal at all. Once you've caught the perpitrator you can talk to the owner more about the issue. If after the second or third catch nothing changes then I would consider relocating the cat into another neighborhood a couple miles away. Someone will surely take in a stray cat.
