June 26, 2012, 5:05 PM — Here's how you can turn off the automatic install of system data files and security updates in OS X Mountain Lion.
1. Go to System Preferences.
2. Click on the Software Update icon.
3. Uncheck the "Install system data files and security updates" checkbox.
Note that this tip assumes that you have OS X Mountain Lion set to automatically check for updates. Mountain Lion seems to default to installing system data files and security updates automatically, so uncheck that box if you don't want that to happen.