How to Remove Activation Lock on iPhone 4

I bought a iPhone 4 from Second hand phone dealer. He sold me that iphone 4 at very low price bcoz of passcode locked. To remove passcode I restored the device with new iOS 7.1. But the iphone is activated with Find My iPhone app.
While activating is shows msg above Login Detail: "This iphone is currently linked to an apple id (x******@gmail.com . Sign In with the Apple ID was used to setup this phone.).

But neither I have any detail of last(preivous) user nor the dealer have. So I tried to activate it with my new Apple ID but shows msg like: Find My iPhone is active on this device, contact old user to remove link of this device from his Apple ID account or enter correct Apple ID info

Please help me to remove this Activation code !!

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