This article is from the Problem Behaviors in Cats FAQ, posted to rec.pets.cats newsgroup. Maintained by Cindy Tittle Moore with numerous contributions by others.
Sometimes there is a problem with a particular cat that fights with other cats. If it is feral, try to make arrangements to neuter it, if possible. If it belongs to a neighbor, try to discuss the matter with your neighbor, and avoid being "threatening." When approached reasonably, most people can be reasonable in turn. Sometimes your neighbor just doesn't know his cat is bothering you.
If the cat actually follows your cat through the pet door, you might try an electronic pet door to keep it out (see Pet Doors).