It seems that at the moment (of course things are constantly changing…) the only way for Free Plan folks to turn off the double optin at the moment is to turn off the great feature of “Collect data from website forms”.
This is found under Settings > Marketing > Forms.
What it means is that any independent capture flows will have to be manually entered into hubspot.
I have to say with this constraint + the lack of control of the timing of the popup chatbot, hubspot becomes seriously less useful. I think to switch back to Drift for their better free chatbot. (The number of conversations plummeted when I switched to chat to Hubspot.) But my company just isn’t big enough to justify Hubspot’s hefty minimum monthly fee.