Expressing finality, that that preceding material is meant as final, not subject to amendation or variation.
Used other than figuratively or idiomatically: A case is closed; specifically, a police investigation or similar is resolved.
Source: en.wiktionary.orgI was a big Nancy Drew reader. Nancy figures it out. Case closed. Sarah Vowell
We will chase terrorists everywhere. If in an airport, then in the airport. So if we find them in the toilet, excuse me, we'll rub them out in the outhouse. And that's it, case closed. Vladimir Putin