Here's how you can disable automatic software update checks in OS X Mountain Lion.
1. Go to System Preferences.
2. Click on the Software Update icon.
3. Uncheck the box next to "Automatically check for updates."
4. Note that this will also disable automatic downloads of available updates, and it will disable the automatic install of system data files and security updates.
5. This will not, however affect the "Automatically download apps purchased on other Macs" option if you have that checkbox checked.