I have gone through the settings and the only thing I can see that would disable the server is network services page on server settings and turn off the service, but that does not get rid of the !m in top menu bar.
Can you please try to do the following?
– Click on the Monitor (the !M icon) in the system tray to open the menu.
– Click on ‘Show the server status’ item to open the Server status GUI.
– Click on ‘Exit the server’.
This will stop all the NoMachine programs, included the !M icon. NoMachine will be restarted when you reboot the machine.