Since version 7.0, the client process has two kind of instances:

i) a main singleton instance, showing the list of available connections and giving access to Player and Server Settings (only Player Settings if the Enterprise Client is installed)

ii) a session instance, launched every time the user connects to a server.

While the session instances can be closed by simply clicking on the X button of the window, the main instance keeps
running unless it is closed by using the tray menu, "Quit NoMachine player" item.

When you want to close a running player, use the tray menu to be sure that all instances are closed.