Normally, if you pinned an app to your “Dock” [the icons at the bottom of the screen], you can simply right-click the app icon and choose > Options > Open at log-in to change this setting.
However, Google Chrome, for some reasons, at the moment, does not have this option checked, but still loaded automatically at log-in.
To disable this, on MacOS Seirra, (also true from Lion on wards), Check the System Preferences > Users & Groups > [your username] > Login Items.
You might see “Google Chrome” in the list of Login Items. By removing it from the list, Chrome should not automatically be loaded anymore.