Minecraft wiki entry for ban, associated text: Message to display to the banned player, to other operators, and in the server logs. If unspecified, defaults to Banned by an operator.
Must be a plain text. Can include spaces as well as target selectors. The game replaces entity selectors in the message with the list of selected entities' names, which is formatted as "name1 and name2" for two entities, or "name1, name2, ... and namen" for n entities.