All great questions!

1) You could add a pop-up that comes up before the search results are shown for flights, with a warning/explanation of basic economy fares. This is successful for us because the user has to at least close the pop-up before moving on and we list the reasons that we do not recommend basic economy.

在我以前的公司,我们添加了pa的警告rt of the custom text on the results screen, at the top, with some things highlighted so it was there for reference. This was nice because the information was right at the top of the screen.

2) I much prefer "show but do not allow" and not hiding them, as I agree with your assessment that the user would then try to purchase elsewhere since it appears cheaper on a different site.

3) That is pretty much how our pop-up works, but there is no proof of acknowledgement.

4) Agreed and there are still some issues there. I believe it is better, though.