By default, the SPA9000 should not ask for any username and password when you open its web interface. In order to reset the unit back to factory defaults, complete this instruction:
Step 1: Using the analog phone connected to the SPA9000's FXS port, dial **** 73738# then press 1 to confirm the reset.
Note: If it still asks for a password then that means this unit is already provisioned by a certain provider. Whoever provisioned the device is the only one who can remove it. Unfortunately, there is no back door.