Originally posted by: shargus
Your original question was "how to restrict the user from changing their password". If you set the minage to the maximum value, the system will not let the users change their password for that length of time, no matter what method they use.
Unfortunately, as karthikraja pointed out, the maximum value for minage is 52 weeks, or 1 year. You might be able to extend it by changing the lastupdate on the password for that account on a regular basis, say, doing a chsec in a cronjob.