Using the Orange School demo site at http://school.demo.moodle.net/ I logged as manager, set changeownpassword to "prohibit." Logged in again as a teacher, I successfully changed my own password.
But, I *think* that is because the teacher also has the role of authenticated user, and authenticated user changeownpassword is set to "allow."
Hmm. If you set it to prevent or prohibit then *probably* everyone but an admin will not be able to change their password.