We currently don't have a way to let you require answers, or trigger an intercept based on exit intent, because the entire ethnio screener often lives in an overlay, which is a glorified pop-up. Even when the screener is being used as a link, and not an intercept, these forms have a long history of annoying people. We don't want to annoy people, and our feeling is that getting an error inside a glorified pop-up or even web form where you're only volunteering for research, can be super annoying. So instead, Ethnio tries to only allow the right potential participants to sign up. Remember we automatically filter responses that have more than three blanks anyway.
The same concept is why we don't allow exit intercepts. That's basically a pop-up as you're trying to leave. Can also be really annoying and borderline spammy. You can also set a delay if you'd like a proxy for exit intent, which at least will show to people after they have performed some amount of interaction on a given page or app state.
If you'd like to filter out responses that are missing any specific questions, you can also do that. So instead of putting the onus on a participant to answer every question, we put the onus on Ethnio and researchers to filter out incomplete responses and only allow the right people to sign up. Using the filter bar under "Responses," it's very easy to hide respondents who didn't answer one or more questions.
Lastly, you could state in the invite page that in order to participate, all or certain questions must be filled out. Especially using the "large formatted text block" which will make the warning text as large as you'd like.