How do I stop the DivX Software update notifications?

You can actually set the DivX Update module to "Never" and it will not check for updates unless you manually request this. 

To do this on Windows, open your Start menu and navigate to All Programs > DivX > Check for Updates. Where it says "Check for updates interval:" use the drop-down menu to select "Never" and click Close. 

On Mac, open your Finder and go to Applications, then open the DivX folder. Double click on the DivX Preferences app. Under the "Software Updates" section, use the drop-down menu to select "Never".