You cannot be serious!
how and where to disable the networkmanger
The same way you installed it. Tell your package manager to remove it.
Or you could just remove it from the startup.
IIRC: redhat systems go like this:
# update-rc.d NetworkManager remove
sudo aptitude remove network-manager
note - the nm-applet will let you set up static IP and manual DNS and all that - rt-click on the applet and select "edit connections" - but I sympathise: I usually prefer manual over gui myself.