This tutorial explains the steps required for disabling changing password for a user on Mac OS X Lion. You can disable by using the option available as part of Parental Control.
Click the Apple icon and select System Preferences from the menu list.
In the System Preferences, navigate to System section click the Parental Controls option.
First unlock Parental Controls by entering the required admin user credential then click the Other tab.
Now mark the check box with label as “Disable changing the password” which would prevent the user from changing their password.