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How do I stop Microsoft Skype For Business from automatically starting every time I rebook macOS?
Did you check the Login Items?
System Preferences > Users & Groups > Login Items
To remove an item, choose it and hit the --button at the bottom. Just checking or unchecking the hide radio button won't disable the login item!
If it's not in the Skype settings, you can always keep apps from automatically starting in System Preferences:
Apple menu -> System Preferences -> Users & Groups -> Login Items.
Simply find Skype in the list and press the [-] button on the bottom.