Man, I could have written this post! Except my friend was the daughter of my mother's best friend in high school. Over the years, the mother and daughter would visit from out of state (her grandmother lived in my state) and I'd have to spend time with her. She was very sweet as a girl, but as she got older, she began to take on some of her stepfather's very bigoted views. When I took her to a dance at my predominantly Latino all-girls high school and she spent the entire night (loudly) criticizing the "unsavory crowd," I knew we had grown too far apart to make a real friendship work. However, her mother loved the idea of her and her best friend's daughter being best friends, and used to talk all the time about the "second generation of best friends" to her, and she soaked up every word. So, like your situation, she thought we were much better friends than I did. She'd write, and I'd compose short notes back. She'd call, and I'd try desperately to make conversation. I can't flat-out cut her out of my life, however, because our mothers are still friends. We are in our 30s now, and this woman and I still keep in touch. About 10 years ago, she came to visit me. It was excruciating, but I lived. Over the years, however, I have communicated a little less. I took a little longer to answer letters or phone calls, and I apologized and cited my busy schedule. After a while, she eased up. We still communicate (we are Facebook friends), but she no longer calls me her best friend and the sister she never had. It made me feel guilty, but leading her on made me feel guilty, too. You can do one of two things: you can start weaning her off you slowly by being gradually less available, but that opens the door to her questioning you about it. That will lead to a direct confrontation, and, I'm sorry, there is no polite way to say, "Gosh, I'm only nice to you out of obligation, so I really don't want to see you, ever." Or, you could just suck it up, do one last act of charity, and tell her, if she ever does plan a trip, that she is welcome. Then, you plan a lot of activities that require little direct conversation, such as movie marathons (double-feature at an old movie house, maybe), local theater productions, or museum or art gallery tours. Then, you can talk about the movie, or the play, or what you saw at the museum. Also, plan a lunch or dinner with some of your close friends, the ones you can tell about the situation and who will forgive you if your friend is terribly negative at the gathering, and it won't be so hard making conversation. Best case scenario, your friends and this woman will spend the whole time asking and answering small-talk-get-to-know-each-other questions, and the time will fly by.
There is a third option: if you really don't care about the pendulum swinging too far in the selfish direction, than why care if you are polite? Tell her to get lost, stop returning her phone calls, and call it done. Then you really won't have to worry about her wanting to visit.