You have to find about a dozen settings, and several of them are under Screen Time. Even when everything is off, the foundation models stay on disk, and that’s about 12 GB. I wrote RemoveMacAI to do all of it in one go. It turns off Siri, Writing Tools, Genmoji, Image Playground, the ChatGPT extension and all the summaries, then it deletes the models and stops macOS from downloading them again.