If you to want to remove a game demo you should look this.
How to Remove Your Game Demo Permanently
choose any you bought game in library -> clilck Support->right click the Pages and Copy page URL
（If you are not logged in, please do it and repeat the steps above ）
The next step is to get the game demo appid .enter the demo properties
（library→you want to removed →rightclick it→properties→update tabs →remember or wrote down the id）
Paste the URL you just copied into your browser，and then to modify your url appid to the one you wrote down ,enter it .
Click on “I want to remove this game from my account permanently”
and then the demo had been removed.
a clean game library is come back, yes!!