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If it was factory reset, as it should be, the first thing it asks is a Samsung account, not the previous owners Samsung account (that's now not associated with your device).
I think you can skip that step, as a lot of people don't set one up, though I find having an extra back up for Samsung related apps useful.
So it should be just waiting for you to either enter your old Samsung account if you had one and remember it, or start a new one.
You can also go to your local Samsung site on a pc and set one up there - then just input the email and password you used on the device.
I can't see how it would only accept the previous owners info.. If it is stuck or frozen, either do a battery pull or do a full factory reset while off using the buttons to access the Recovery Mode screen.
http://forums.androidcentral.com/showthread.php?t=298808
On Edit: Sorry, you said a Galaxy S6 so you can't pull the battery but there should be long press option for the Power button. It sounds like you have already tried Recovery Mode. Please register to reply and someone with an S6 can walk you through if needed.