How to clear an ASA`s configuration
You may find that there is a time in which you haven't got access via the standard ASAOS CLI to change, amend or edit your current configuration.
In this example we will show you the steps required for removing the configuration via ROMMON mode.
Here are the steps :
1. Reboot the device
2. On boot hit `escape` so that you break the normal startup process and enter ROMMON.
3. Change the current confreg so that you can bypass the current startup config sing the command:
rommon #1>confreg 0x41
4. Reset the appliance with the boot command,
5. You will then be presented with the ScreenOS CLI and the ASA having a completely clear config. Run the commands :
ciscoasa > en [when asked for password just press enter]
ciscoasa # conf t
ciscoasa (config)# config-register 0x01
ciscoasa (config)# copy run start
ciscoasa (config)# reload
6. Your ASA will now be rebooted and the device will be booted into a blank configuration.
Reference : http://ezinearticles.com/?Password-Recovery-on-the-Cisco-ASA-Security-Appliance&id=664795