Prevent Steam from Opening Itself on Mac
The simplest way to prevent auto-launching of Steam app is to remove it from the Login Items app list of OS X, this is set at a user level, meaning if you have multiple user accounts on a Mac you’d need to repeat this same process for each user on that Mac.
- Go the Apple menu and choose “System Preferences”, then go to “Users & Groups”
- Select the user that is active in OS X, then choose the “Login Items” tab
- Select “Steam” from this list and then hit the Delete key on the keyboard to remove steam from the automatic launch on login list
- Close out of System Preferences
The next time you log into that user account or reboot the Mac, Steam will no longer open itself. Instead, if you want to launch the Steam client, you’ll need to open it yourself from the /Applications/ folder, like any other app launched the usual way.
In Login Items this goes for any other application you want to remove. :)