application

OOTB you can use the 'Maintenance Window' feature in the Admin Console but this will apply to the whole environment, not to a specific Application. Otherwise you can do exactly as [mention:126a676c85024855948336691b0a7b92:e9ed411860ed4f2ba0265705b8793d05] describes, to toggle the value of a constant which is used to hide the default landing pages for all of the roles involved in your application and replace with a blanket "This Application is undergoing maintenance" message or equivalent as the only User Interface they can see.

Apart from what both of these people said, if you are using tempo by any chance (that I assume you are not), then you can also unpublish the application. That should take all the records/actions away from tempo.