You are embarrassed by that game in your Steam library. Don't feel ashamed. I won't say its name. Nobody needs to know.
You've tried hiding that game in a folder, but a folder isn't enough. You want it gone forever. You feel guilty. That game is still hiding in plain sight, like a smutty magazine buried in your living room bookshelf.
I have good news. Steam will finally allow you to remove games permanently from your library. NeoGAF user Enter the Dragon Punch posted the following instructions this morning, after finding the option in the Help menu. Here are the steps.
Select "Steam Support"
Select "Games, Software, etc."
Select that game
Select "I want to permanently remove this game from my account"
Select "Ok, remove the listed games from my account permanently"
And it's done! You've removed
Aliens: Colonial Marines that game from your Steam library.