As far as I can tell, a person is free to go if they refuse screening: They won't be getting on a flight, but they can just leave. There's no detainment involved in this process.

Whether they can then elect to stay is a different matter, I think.

But so what? How long would a person have to stand in a screening area before someone who properly represents the ownership of that space shows up and authoritatively tells them to GTFO, do you suppose?