In the category settings, just under the place where you set the email address, there should be a Accept emails from anonymous users with no accounts setting. In my testing that allows me to send an email that isn’t registered to a private category.
Unfortunately, it appears this doesn’t work for emails that are registered:
Maybe form templates would help? It would mean duplicating the Typeform survey (maybe imperfectly), but people would not be able to create free-form topics.
Is this too big of a problem that it can’t be solved with social pressure? Maybe just remove topics that don’t have the right format and send them a message about how to do it the right way.