Before anything, if Steam is open, close it completely. Not doing this can cause strange behavior that might affect performance, or damage the computer itself.
1. Toss your Steam app into the trash. This won’t remove anything on its own, but it’s a good first step. You can do this by dragging the icon from your Applications folder to the Trash icon, or by right-clicking the icon and selecting “Move to Trash.”
2. While in the Applications folder, press Command + Shift + G. This will open a pop-up that asks you what folder you want to open. Enter “~/Library/Application Support” (without the quotes) and click “Go.”
3. In the new folder that opens, find the “Steam” folder and move it to the Trash.
At this point, dragging this folder as well as the Steam application to the Trash will delete all of your Steam data.
If you’d like to save your game data, in case you decide to reinstall Steam in the future, delete all of the files in the “Steam” folder except for the SteamApps sub-folder.
Keep it there on your local hard drive, or copy it to an external drive, and move it back when you’re ready to reinstall Steam.
Either way, you’ve now successfully deleted Steam on your Mac.