There is a toggle on https://admin.mistral.ai that allows you to disable training for both Vibe and Console/API for your entire organisation. And I'm not on the enterprise plan. I've disabled training the first time I created an account, and it has remained that way.
You story is also very confusing due to the wording around "opt-in by default" and "disappointed [about] opt-in to training" (most people would be disappointed about an opt-out) and probably conveys the wrong message to most people.