Automatic software updates are a system wide configuration, so they can't be configured in the client's CFG file or in any of other user side configuration files.

Automatic updates can be disabled via User Interface in:

Settings -> Player -> Updates for client installations

Settings -> Server -> Updates for server installations like NoMachine Free Edition

and unselect 'Automatically check for updates'

If it's preferable to not use the UI for some reasons (e.g. avoiding that each user needs to configure by himself/herself such setting), there are some alternative possibilities.

1) If NoMachine clients run in a controlled network, the system administrator can block the pool of software update server IPs listed in the 'update.cfg' file:

/usr/NX/etc/update.cfg on Linux

C:\Program Files (x86)\NoMachine\etc\update.cfg on Windows or, if only the client is installed:
C:\Program Files (x86)\NoMachine Enterprise Client\etc\update.cfg

/Applications/NoMachine.app/Contents/Frameworks/etc/update.cfg on macOS

2) If the user must never be able to update the client, even if he/she has administrator's access, removing the file 'update.cfg' from the NoMachine installation on the user's computer is a way to prevent the software update check  (since NoMachine won't know any longer where the update servers are located).

3) If you prefer a configuration keeping intact the possibility to run a software update in the future, you can edit the following file:

Linux: /usr/NX/var/db/update
Windows: C:\ProgramData\NoMachine\var\db\update
macOS: /Library/Application\ Support/NoMachine/var/db/update

and set '0' to the second value of this string (replace '1' with '0'):

0:1:0:0:0:0:0

The value '1' (visible in the example above) is the default value.