Why exactly would you want to remove your Domain Administrator, or any Clients for that matter, ability to change their password. I highly discourage this type of activity. Preventing users from that ability leaves you open to attacks, should anyone aquire the password. Myself, I prefer to change mine at least once a month to prevent any possible attempts on my machine(s). But that's just my preference. I'd still highly discourage the type of action you're wanting to set up.