Google Nexus S i9023 hard reset Nexus S Recovery Hboot
With the i9020 Nexus S turned off. Press VOLUME UP + POWER buttons.
When you see the exclamation mark and the Android, you should hold down the power button and then press the volume up key, while still holding the power button. Don't press them both at the same time.
In recovery menu select 3rd line Recovery with Volume down button then press power to confirm Nexus S factory data reset.
in detail based on comments
1. Remove Battery
2. Press Vol. up + Power Button.
3. you will see a menu...with vol. down button, come in RECOVERY then press Power Button...
4. It will restart and you will see Android logo with triangle.
5. Now keep pressing Power Button then click once Vol. up button.
6. you will see another menu, now come in Factory reset with Vol. down key and Press Power to continue....
7. now press DELETE ALL DATE and phone is Hard Resetted.
or in menu
Tap Settings > Privacy > Tap Factory Data reset > Reset phone > Erase everything
On Andorid ICS 4.0 Select Settings → Back up and reset→ Factory data reset → Reset device→ Delete all.