There is a setting on Gemini paid, however it isn't about disabling model training, only activity storage length.

Also (On manual deletion):

"Even when Gemini Apps Activity is off, your conversations will be saved with your account for up to 72 hours to allow Google to provide the service and process any feedback."

And in any case:

"Conversations that have been reviewed or annotated by human reviewers (and related data like your language, device type, location info, or feedback) are not deleted when you delete your Gemini Apps activity because they are kept separately and are not connected to your Google Account. Instead, they are retained for up to three years."