I have done it in the past by modifying some files, but that was on a really old version. I am not up to version 4 yet, so it's unlikely the same files would still apply. Also, you need to hide the "Forgot Password" links so they can't reset their password that way either. What you can do is search for the phrase Password in the language properties files to find the key used for that on screen label. Then search the source code for that key and you'll find code that allows for changing their password. Try removing those lines or commenting them out (on a test system first of course). That's all I can offer without the source code.
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