Simply right-click on tile of the game you wish to hide in your library and select Hide from the menu. This will not remove the game from your account, it will only change the display location in Uplay.
All hidden games can be found in the Hidden tab at the bottom of your games library.
If you wish to reverse the process, navigate to your hidden games, find the game you want to be included in your library again and right-click on it. Choose Unhide from the menu.