How do I make Update Manager stop opening automatically?

This may work for you. I haven't verified if it works in GNOME.

• Open the Software Sources application.
• Select the "Updates" tab.
• For the "Automatically check for updates" option, select Never.

After this, you will have to open the Update Manager application manually.

I found this post with the following instruction:

open gconf-editor, browse down to Apps -> Update Notifier, and uncheck the auto_launch option

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OK, try this one. It's similar to the other answer but disables it instead of deleting it entirely. It might be safer for you to open the file in an editor and change the line by hand instead of the sed command that post uses.

The procedure, using an editor:

1. Open a terminal
2. Move to the autostart directory (cd /etc/xdg/autostart) and list the files there (ls).
3. Find the startup program you want to run (in our case update-notifier.desktop) and open it in an editor (gksudo gedit update-notifier.desktop). Enter your password if necessary.
4. Be careful not to change anything except this line. Find this line:
   NoDisplay=true
   and change it to:
   NoDisplay=false
5. Save the file and exit.
6. Reboot to make sure it worked.

In Precise, it is done using dconf now rather than gconf.

1. Install dconf-tools

   $ sudo apt-get install dconf-tools

2. run dconf-editor and navigate to com > ubuntu > update-notifier.

3. Tick no-show-notifications

And here's how to do it from the command line:

dconf write /com/ubuntu/update-notifier/no-show-notifications true