With the open skype application, follow the next steps:
- Go to Tools> Options.
- Highlight the General configuration of the section in the upper left.
- Uncheck the Skype Start option when I start Windows on the right.
- Click on the Save button.
On the other hand, if this is the Skype of UWP application those options will not be available. Based on a similar question on the Microsoft Answers website, your Skype Community Moderator explains what to do:
- The new Microsoft of Skype for Windows 10 (skype UWP), the application has never been present in the Home tab. Windows applications are not managed in the same way as a Win32 application. The classic Skype for Windows Desktop is a Win32 application, Skype UWP is not.
- If you close your computer without leaving the Skype session UWP the application, then at the next startup of the computer, skype will automatically run in the background. To maximize UWP skype you will only have to click on the application icon.
- If you prefer not to be signed in automatically on Skype for Windows 10, you can exit the application. It is not going to log in automatically after that.
In other words, UWP skype users simply need to log out of the application.