The best way to stop someone from getting a culture victory in Civ 6 is to focus on building Wonders, Great Works, and other culture-generating buildings. You should also try to build as many districts as possible, as they will generate culture as well. Additionally, you should try to build as many trade routes as possible, as they will also generate culture. Finally, you should try to keep your borders open to other civilizations, as this will allow them to contribute to your culture as well.
