In Arc Catalog (10.5.1), when right-clicking on a SDE geodatabase connection, selecting properties, connection tab, show connected users, one can select ALL of the connections and right-click -> Disconnect.
In Pro 2.1, same steps, however once you right-click (after selecting all of the connections), the selection is lost and you are only able to disconnect whatever connection the mouse pointer happened to be over.
Forcing disconnects is an essential part of enterprise geodatabase administration. Many of my geodatabases can have dozens of connections. Having to right click on them one by one.....