1. Click on settings in the top right corner in Skype for Business.
2. Click on personal and uncheck "Automatically start the app when I log on to Windows" option.Then click on OK.
3. Close Skype for business by clicking on 'X' on the top right corner.
4. Now go to bottom right corner of your Windows machine and click on the arrow shown below.
5. Right click on Skype for business icon and click Exit.This will stop the Skype from running in the background.