August 2016
You will automatically log in to your EA account when you log in to the Gamertag that you've attached to this specific EA account.
Log out of your Microsoft account / Gamertag as detailed in the following article:
http://support.xbox.com/en-IE/xbox-one/console/how-to-sign-out
If you then want to login to a different EA account then you also need to setup a different Gamertag.