It is not possible to disable pop-up or "balloon" notifications. Balloon (pop-up) messages are a central security feature of NoMachine software. They are used not just to notify the local user that someone has connected/disconnected, but also prompt the desktop user to accept a specific function (e.g accept or deny an incoming connection). Whilst in version 7 it was possible to disable all balloon messages using the appropriate key in node.cfg, from version 8 the disabling of all balloon messages is no longer possible, for the simple reason mentioned before: the notifications are there exactly to inform the local user, or anybody connected, that there is now somebody else looking at what you, the user, are doing. 

For companies which are concerned about showing sensitive information such usernames and IP addresses of corporate machines in the notifications, from version 9, it is possible to configure them in "privacy mode" so that all interfaces where user names and IPs are visible can instead show generic messages e.g., “A user is connecting to your machine”.

On the server, this new key has to be added in node.cfg: 

# 
# Configure which information will be displayed in the notification 
# dialogs issued when a user connects or disconnects. Accepted values
# in a comma-separated list are: username,userid,ip. They will show 
# respectively: the name of the system user, its User ID on NoMachine
# Network, when connected by this service, and the IP of the machine 
# of the connecting user. By default they are always displayed.
# 
# ShowNotificationInfo "username,userid,ip"

To remove sensitive information from the pop-up notifications, uncomment the key (remove the pre-pending # symbol) and set it to:

ShowNotificationInfo ""