Occasionally one has to violate best practices to clear password history.
AIX, and Linux:
chuser histsize=0 username
Sometimes on AIX you have to make sure these lines exist too.
edit this file so that root has a minage and histexpire:
vi /etc/security/user
minage=0
histexpire=0
Solaris:
chmod 600 /etc/security/passhistory
vi /etc/security/passhistory (delete the line for your user here)
chmod 400 /etc/security/passhistory