Prevent linux user from changing their password in ssh

Question

How can I prevent users from changing their passwords? I still want to be able to change the passwords as root if necessary but keep the user from changing their password.

Answer 1

Do chmod go-rx /usr/bin/passwd Normal users can then not run passwd. If you want some users to be able to, you can put them in a special group perhaps.

Answer 2

passwd -n 9999 user will prevent user from changing his password for almost 274 years.

If you want to have passwordless user, which is unable to change his password, open /etc/shadow as root, find the line which begins with the name of the user, and change the content between first and second colon to U6aMy0wojraho.
^{(source: https://help.ubuntu.com/community/PasswordlessGuestAccount)}