How do configure Skype For Business to not start when macOS starts?
Check System Preferences - Accounts for Sierra, Users and Groups for Catalina - Login Items and make sure it's added there. 

Alternative 1: 

Open the context menu on the icon in the dock -> options -> uncheck "open at login".

Alternative 2: 

Open the terminal and type launchctl list | grep skype. 

Then use sudo launchctl remove &lt;name from the previous output&gt; to prevent it from starting at login.