If you do not wish to use the Network Manager, you have two options: disabling it or uninstalling it.

Option #1: disable it (recommended)

To stop the Network Manager, open a terminal and run:

sudo service network-manager stop

You can restart the Network Manager at any time by running:

sudo service network-manager start

To prevent the Network Manager from starting on boot, create a file called network-manager.override on /etc/init/ and add "manual" to its contents. This can be done with the following command:

echo "manual" | sudo tee /etc/init/network-manager.override

To have the Network Manager start on boot again, just remove the file you generated:

sudo rm /etc/init/network-manager.override

This is the option I recommend as it is easily reversible and keeps the Network Manager installed but inactive. If you run into trouble when trying to connect to the Internet, you can easily start the Network Manager and spare yourself from lots of pain.

Option #2: uninstall it

If you are sure you will never use the Network Manager again, you can uninstall it by running:

sudo apt-get remote network-manager network-manager-gnome

To stop a possibly running instance of the Network Manager, run:

sudo service network-manager stop

NOTE: if you uninstall the Network Manager and stop it, you will immediately lose Internet access. Make sure you do this only if you know how to configure your network interfaces and connect to the Internet via command line (without Internet access you will not be able to reinstall the Network Manager with apt-get). If you do not know what I mean by that, use option #1 instead.