restore factory settings in my iphone 4 which is actually second hand.

I want to restore factory settings in my iphone 4 which is actually second hand. however, I cant because it has an apple ID of the old owner and i do not know the password. When i select erase all content and settings, the email ad of the previos owner appears and is asking for its password which i do not know. I have already change the email account in its itunes and appstore to my email ad. But when i go to the erase all content and settings, it still has the old email ad of the previos owner. what should i do? thanks.