To prevent a user account from being used, it’s often simpler to lock the account, rather than changing the password, deleting it or otherwise disabling it. The easiest way to do this is with the getprpw command.
getprpw returns a series of bits, check man page for meaning:
# /usr/lbin/getprpw [ -m lockout ]
To reactivate a locked account:
# /usr/lbin/modprpw -k