Hard Reset SAMSUNG I777 Galaxy S II

First method:

The device must be turned off so hold down the Power key for a few seconds
After that, press and hold together: Volume Up + Volume Down + Power button for a couple of seconds.
You should release Power key when the Samsung Logo appears on the screen.
Then select from Recovery Mode menu "wipe data / factory reset" using Volume buttons to navigate and Power button to confirm.
Choose "Yes -- delete all user data" to confirm the whole operation.
When the Recovery Mode appears again select option "reboot system now".
Well done! The hard reset has just been performed.