Sometimes its quite uncomfortable while you are in a call on Skype, and your friend shares his screen with you, and it gets opened automatically, so if you want to decide on your side either you want to open shared screen or not then guide below works for you. This option of doing so is only included to Skype Metro for Windows 8.
Disable Automatic Screen Sharing in Skype for Windows 8
From Start screen click on Skype to load it and then press “Windows” + “I”
Go through “Settings” > “Options” > “Incoming video and screen sharing” and select the option “Ask” from its related drop down list which has by default setting to “Start automatically”
Now, whenever your friend on Skype want to share screen, a pop-up message will appear on your screen where you can make a choice to display shared screen or not.