Disable Password Change

You could effectively do this by increasing the local password policy minimum duration setting to a very high value. However, you know this is very bad practise from a security perspective right?

Menu path: System > Users > Authentication

Tmsh command path: [tmsh] modify auth password-policy...

If you are talking about APM, then you can switch from AD Auth action to LDAP Auth, as LDAP Auth does not support password change feature.