How do I disable network manager permanently?

The method depends on desktop environment:

• For Ubuntu MATE 18.04 LTS and 20.04 LTS purging network-manager package is safe. You can simply run:

  sudo apt-get purge network-manager
  

• For Ubuntu 18.04 LTS and 20.04 LTS with GNOME desktop purging network-manager package will also purge ubuntu-desktop and gnome-control-center (essential part of GNOME desktop). So it is not an option.

  Here you should disable NetworkManager service (as you have already done):

  sudo systemctl stop NetworkManager.service
  sudo systemctl disable NetworkManager.service
  

  and three more services:

  sudo systemctl stop NetworkManager-wait-online.service
  sudo systemctl disable NetworkManager-wait-online.service
  
  sudo systemctl stop NetworkManager-dispatcher.service
  sudo systemctl disable NetworkManager-dispatcher.service
  
  sudo systemctl stop network-manager.service
  sudo systemctl disable network-manager.service
  

  and then reboot.

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Notes:

1. You can read more about network configuration with /etc/network/interfaces from Ubuntu 16.04 LTS Server Guide.
2. Modern Ubuntu 18.04 LTS server uses netplan, you can read about it in the Ubuntu 18.04 LTS Server Guide.

Try the mask command:

sudo systemctl stop NetworkManager.service
sudo systemctl mask NetworkManager.service

mask NAME...

Mask one or more units, as specified on the command line. This will link these unit files to /dev/null, making it impossible to start them.
This is a stronger version of disable, since it prohibits all kinds of activation of the unit, including enablement and manual activation. Use this option with care. This honors the --runtime option to only mask temporarily until the next reboot of the system.
The --now option may be used to ensure that the units are also stopped. This command expects valid unit names only, it does not accept unit file paths.