Sure. In your library, hover the mouse cursor over any game you've added to a collection. Four buttons will appear. One of them says "Remove". This doesn't work on Things you own, or Things you've rated, though.
Is it a game you claimed with a download key? Removing it might require sending an e-mail to support@itch.io for now. May I ask why you don't want it anymore?
Because they were giving a combo of 18 free games but then for the trouble I only got 5, and when I check that this is the game I did not like them at all in my library ...
