When a computer has been published on NoMachine Network, it is possible to restrict remote access by using the 'availability list' feature. The computer must have NoMachine v9 installed and must be published on Network to be able to limit access to specific NoMachine Network users. It is not necessary to be logged in to do this. Access the 'Change machine availability' panel on the NoMachine server you want users to access: Settings > Network > Machine > Machine Info > Available to and clicking Change. Add the User Ids of the users that are allowed to remotely access the computer over NoMachine Network, click 'Apply'.

If you are already logged in and wish to limit access just to you, select 'Make this computer available only to <your_Userid>. This option appears only when you are logged in.
 

Note: You don't have to be logged in to Network in order to configure the machine's availability to Network users, but logging in first will give you access to your own Network Users book. When configuring the availability list, NoMachine will show you the User Ids from your Users book, so that you can add them in one click.