The Steam application is required to play all games purchased through Steam, as well as retail games registered to Steam.
The following instructions will describe how to remove Steam completely and then your games. If you complete all the steps and decide to reinstall Steam later, you will need you account name and password to reinstall your games.
If you are using Lion press CMD+G and type ~/Library to reach your Library folder.
With the Steam application active, click the Steam menu and Quit Steam to exit Steam.
With these files deleted, you can perform a reinstallation of Steam if you are troubleshooting a problem. The steps above complete the removal of Steam and your games from your Macintosh.
If you would like to avoid re-downloading any of your currently installed games, make sure to backup your SteamApps folder. You can find it in the following directory: Hard Drive>Users>[Your Mac Username]\Library\Application Support\Steam\.