Go to your AO3 page and click the Edit Works button at the top of the page. On the Edit Multiple Works page, click the All button on the top right, and then Edit beneath it.
There will be a list of all your fics, and below that a list of all the edits you might want to make. Any edits you make will override the current tags/data; any fields you leave blank will not change anything.
In the bottom box, one of the sections is “Visibility.” Tick “Only show to registered users.” Then scroll to the very bottom and click Update All Works.
However, I’m not sure how much use it will be, given that the AI is already trained; they might be doing further scraping for data, but they might already have gotten everything they want from AO3.