One possibility would be to remove the package which includes the update notification utility. In Mandriva 2009.0 this package is mdkonline. Here is the description from rpmdrake:
mdkonline - Mandriva Online Update Tool
The Mandriva Online tool allows users to be kept informed about security updates, hardware support/enhancements and other high value services. The package include :
* Update daemon which allows you to install security updates automatically,
* A KDE/Gnome/IceWM compliant applet for security updates notification and installation.
Note: mdkonline provides an update notification utility and (AFAIK) does not automatically download or install updated packages. It does (however) check for and download the database information for urpmi, and pop up a message if updated packages are available. I find this service to be very useful, but your situation may differ.