I just went into my settings after reading this article (and getting a bit frustrated that it was a wall of text with no clear instructions right up front).
I did a search in settings for "gemini" and found the Gemini settings. There's no clear 'off' switch. But there's is an option to switch back to Google Assistant. So I did that. And not when I search for Gemini in settings it urges me to enable Gemini.
I'm taking that as a good sign that I've disabled it permanently. Thought I'd post this info in case it helps someone else. Or so if I'm incorrect somebody else can correct me.