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?
This can be done from within the Steam client
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.