Just stop it from running in the background. You can remove it from your Startup items or just close it from your system tray. If you're running Windows 10, just bring up the Task Manager (CTRL/ALT/DEL) and in the Startup tab, check to make sure it is not listed. If it is, just click on Disable. In your system tray, click on the little "up" arrow down on the lower right. Find the Steam icon, right click on it and select Exit.
I also use and love WinPatrol, but it isn't necessary in this case if you don't have it.