How

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.

see

https://askubuntu.com/questions/1091653/how-do-i-disable-network-manager-permanently