Every time I boot into OSX or Windows, Steam automatically starts.
Is there a way to stop Steam starting whenever I boot up my computer?
There is an option within Steam that toggles whether or not Steam starts with your computer. To change it, open up the main Steam window and select 'Preferences' from the 'Steam' menu, followed by selecting the 'Interface' tab, which brings up the following screen;
Simply uncheck 'Run Steam when my computer starts' to prevent Steam from starting with your computer. (Obviously if you wanted to Steam to start with your PC, you'd check this box instead of unchecking it!)
If, on macOS, this doesn't work for you, check for Steam in "System Preferences" > "Users & Groups" > [Your user] > "Login Items", and remove it via the '-' sign.
An alternative solution on Windows (starting from 8) is to open task manager and disable Steam Client Bootstrapper from the Start-up tab.
Note: This is an override switch to Steam's setting. You cannot re-enable it from Steam settings after disabling it here.
In Windows 7 click the Start menu and type msconfig
in the Search box. In the System Configuration program that opens, click the Startup tab, and then uncheck Steam. This does pretty much same thing as the Windows 8 tip.
OSX = open steam then next to apple icon in top left corner steam > preferences > interface then uncheck - 'run steam when my computer starts'
For Mac users:
open /Users/yourname/Library/LaunchAgents
command and remove valve related plist file.