You can use Profile Manager in OS X Server 2.1 and later to enable Software Update-only mode in the Mac App Store on OS X Mountain Lion and later.
Choose "Restrict App Store to software updates only" in the OS X Restrictions payload for a user, group, device, or device group.
Note: With this enabled, users will only have access to the Updates section of the Mac App Store. All other sections will be disabled (dimmed). This will allow users to install OS X updates, but not apps, app updates or major releases of OS X.