Switch server key to the maintenance mode with the AIX installation CD and boot into maintenance mode or do a hard reset. Then access the rootvg from the CD and start a shell. You can modify the "/etc/security/passwd" file with vi by removing the encrypted password after the "=", leave the rest of the information intact and save the file.
The root section of the passwd file should look similar to this:
root:
password =
lastupdate =
flags =
Before you shut down the server, type:
sync; sync; sync
and press return.
To restart the server, press the reset button and move the key to the normal mode.