by Hard Reset
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.
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.
Before you buy a new cell phone or new tablet, please try the following procedure to repair your device. First charge your battery, backup your important data if possible and in most cases take out SIM and SD card before factory reset. It is impossible to recover your data after doing a hard reset, so online backup is always important. For all of your data, a backup should be done on an external device, hard drive, raid system or media. With or without insurance, if your cell phone lost, damaged or stolen at least your invaluable data is backed up in a safe place. If you are about to sell your phone and you want to wipe your personal data, or if your phone affected with virus you have to do a hard reset. In some cases, you can delete the forgotten password or lock pattern from your device as well. All information is for devices with stock based ROMs. If you are using rooted device, always back up your EFS folder first, containing your IMEI info, etc.!