In order to hard reset a Nokia N900, you should perform the following steps.
Remember! Hard reset will delete all the data on your device; pictures, contacts, messages, apps and more.
Nokia N900 device will be brought back to the default configuration of a brand new device.
1. Start by press-holding the Power button until the phone turns on.
2. Open Settings.
3. Tap on the screen's upper corner.
4. Select either Wipe or Factory Reset.
5. Tap in the device user code. Confirm.
6. Voila! Done.