Steps to  block updates to Mac OS X Catalina:

• Open System Preferences by clicking the Apple logo in the upper left corner.
• Go to the App Store and select it. On macOS, you'll now see a number of options for automatic updates.
• To turn off automatic updates, uncheck the box next to Automatically check for updates, which is selected by default.
• Click the empty box next to Automatically check for updates to enable automatic updates. A tick mark will now appear in the box.

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