We have had this problem, and dealt with it by lying in wait for the Evil Neighbor Cat and squirting it with a squirt gun. You put a little bit of vinegar into the water in the gun—it won't hurt the cat; it was a vet who suggested that to me, so I'm sure it isn't harmful. We did that a few times, and the cat never came back. Unfortunately, *other* neighbor cat's did, and we had to repeat the exercise.
Also, we found we had feral cats coming in. The town where we lived had an animal control office which would loan residents cages for trapping feral cats, at no charge (though this was some time ago). Once we trapped a 'wild' cat, we would take it to the animal control office, where they would determine whether it was adoptable, in which case it got its shots and went to the pound, or that it was un-adoptable, in which case it was put down. They did find it hard to believe that on the form we filled it, under "Where did you trap this animal?" we had entered "the kitchen", but we convinced them in the end.
We had this problem at several different residences on two different continents. We finally gave up and shut the cat flap at night, because we couldn't prevent 'new' strays from snacking while we were asleep.
At one place where we lived, OUR cat went next door and ate their cat's food, even though it was the same flavor of the same brand that our cat god at home. Fortunately we were on good terms with the neighbors, and they didn't mind, so we bought them a 20-pound bag of cat food every year (over their protestations that we didn't have to) and all coexisted peacefully.
...All I can say is that my life is pretty plain.