Q: Delete Beta Updates of OS X in App Store

Back up all data.

These instructions must be carried out as an administrator. If you have only one user account, you are the administrator.

Please triple-click anywhere in the line below on this page to select it:

sudo softwareupdate --clear-catalog

Copy the selected text to the Clipboard by pressing the key combination command-C.

Launch the built-in Terminal application in any of the following ways:

☞ Enter the first few letters of its name into a Spotlight search. Select it in the results (it should be at the top.)

☞ In the Finder, select Go ▹ Utilities from the menu bar, or press the key combination shift-command-U. The application is in the folder that opens.

☞ Open LaunchPad and start typing the name.

Paste into the Terminal window by pressing the key combination command-V. I've tested these instructions only with the Safari web browser. If you use another browser, you may have to press the return key after pasting. You'll be prompted for your login password. Nothing will be displayed when you type it. Type carefully and then press return. If you don’t have a login password, you’ll need to set one before you can run the command. You may get a one-time warning to be careful. Confirm. You don't need to post the warning.

If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator. Log in as one and start over.

Wait for a new line ending in a dollar sign ($) to appear below what you entered. You can then quit Terminal.

Terminal.app

sudo defaults delete /Library/Preferences/com.apple.SoftwareUpdate CatalogURL

or manually delete the key 

sudo vim /Library/Preferences/com.apple.SoftwareUpdate.plist

You, Sir, are to be applauded for providing a simple working fix for an irritating situation.  Kudos to you.

Thank you, good sir!