Help! How to remove previous owner's iCloud Account?opipi

I Bought an iPod Touch 5 th gen from a store but found that the previous owner did not remove his iCloud account. According to my research, the only way to remove it is by the person himself. I tried hard resetting but is till does not go away. His iCloud account prevents me from puttingy own iCloud or updating the iOS. I emailed the email shown for the person's Apple ID when the device login but he never replied or did a thing. Is there any other way such as reinstalling the firmware?