“Sew buttons on your underwear!”
This is the correct response when someone answers with “So?”
We used to respond with “So there.” but my ex-husband taught me what they say in Wisconsin. He learned it from his Mom.
(I looked up the meaning; In the Depression people were very poor. When the elastic wore out on their underwear, they would sew a button on it and fold it over.)
This will confound any speaker and guarantee you getting in the last word.