This isn’t a connection/session, it’s a computer on your local network. You can recognize it from the little icon. Compare it for example to a session icon: see figs 3 and 4 in this tutorial: https://www.nomachine.com/finding-other-nomachine-computers-on-the-same-network. Just like you can disable “being discovered”, you can also disable discovering other computers on your network. Check the box “Don’t show other computers on the network” that you can see in the screenshot (Open the NoMachine connection GUI, click Preferences and then Appearance).