Samsung Google Nexus S 4G phone hard reset
With the Nexus S turned off Press VOLUME UP + POWER on the phone.
In the Recovery menu select (with Volume DOWN) RECOVERY, then press POWER.
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.
Nexus S will reboot, After Google logo Android appeares. If it won't reboot after a few minutes, just take out battery, then reinsert it.
On Andorid ICS 4.0 Select Settings → Back up and reset→ Factory data reset → Reset device→ Delete all.