Here is how to hard reset the unit:
Press VOLUME UP and DOWN then power on until you see the two choices on upper left of the screen.
If you can't make it, just press between volume and and down and power on. It must be at the same time.
Then if you see two choices unpress it, and press volume up again as your choice. Then it will show Android system recovery utility. Select wipe data recovery as shown in the image below. To select the wipe data recovery use the voume down to select it, then press the left side of the home button.
Select Yes delete all user data.
You will see that the system is wipping now. Then after wipping, just select reboot. The unit will restart now and will stuck on logo for a few seconds. That's normal, just wait for a while. After that VIOLA!
Just watch video below for more clearer tutorial. Enjoy!