People write horror stories in non-horror fandoms. They also write highschool AUs, and alien invasions, and all sorts of things that take the characters outside of the canon genre. I don't generally like those things, so (if it's not an author I really like) I avoid reading them. It's not super difficult. The key is, rather than reading them and then commenting to tell the author you didn't want to read them, you simply don't read them.