Disable Ubuntu Update Manager's new version warning

Open the /etc/update-manager/release-upgrades with your favorite text editor, ie:

sudo nano /etc/update-manager/release-upgrades

Change the Prompt= line from Prompt=foo_setting to Prompt=never.

If you want a graphical way of doing it open the Ubuntu Software Center and under Edit > Software Sources > Updates you will find a Notify me of a new Ubuntu version option, from the drop box select Never.

Or just use sed! We just added this to our setup script to avoid our users from borking their own systems by doing upgrades.

sudo sed -i 's/Prompt=.*/Prompt=never/' /etc/update-manager/release-upgrades

You can also just disable checking altogether by doing apt-get remove ubuntu-release-upgrader-core