I have not kept up with the terminology, but for a while I heard people use the term "Just no" to reject something that is so horrible that they shouldn't have to explain why their saying no.

Like if a friend offers you a cold drink and you say "No" you might expect push back. "Aren't you thirsty?"; "It's hot and you should drink something!"; or "Isn't this your favorite drink?"

But if someone offers you food that fell in the toilet "No. Just No" implies the "no" is so severe and so final that no further explanations or excuses should be required.

I think it means "Just No _____ -in-laws" are so horrible that no explanation as to why we can't stand them is necessary (but we will still type up 8 pages to explain our position).