Chuck’s answer to “Posted front and center above my computer, amongst other Wendigan pearls of wisdom, is the "Finish …” > Likes and Comments

I take the South Park excuse instead of not finishing a story. Early on, if Matt and Trey couldn't write themselves out of the story they started, they brought in the Crab People.

I just say fuck it and add a page break, then usually have Jules from Pulp Fiction come in and kill everyone, or Magneto suddenly takes over the world. The End.

If I later think of an ending, I finish later. Else, Magneto wins.

Or just stick the idea on the shelf for later. Orson Scott Card did that with one of his early books and when his agent called up saying. "I just sold your triology" when he had only two books that idea got revived as a third book.