How can I completely remove NetworkManager?
First edit /etc/network/interfaces so that the ifup utility can be used to configure eth0 once NetworkManager is gone.
Remove NetworkManager from the system
sudo apt-get purge network-manager
Configure eth0 using ifup.
sudo ifup eth0
Gnome:
sudo apt-get remove --purge network-manager-gnome network-manager
KDE:
sudo apt-get remove --purge knetworkmanager network-manager
Unity : lol, Unity.
It might be worth it to disable it, configure your alternate setup, and then uninstall it.
Stopping and Disabling NetworkManager | help.ubuntu.com