If you do not want to avoid but to try to clarify this issue with those who are obsessed with or just concerned about the flat earth theory rising mostly from primitive interpretation of the bible, Prof. Dr. John Lennox, a habilitated mathematics professor at the University of Oxford, gives you some powerful arguments. See his book: "Seven Days That Divide the World"

He wrote a lot about scientifically correct (and by the way christianity/religiosity supporting) interpretations of biblical texts. He did amazing work there! :)

Best,
mountain.phil28