I haven't seen a mod for this. You would need to write some code to do it. It would probably be easier to just not allow them to change the email addy (you'd have to remove all instances of it in the templates).
The fields in the form where they are allowed to type in a new email addy.
Actually, try this on a test site or a test style first to make sure that will work. It may be that you need to make the input field hidden instead of delete it. I can't remember if it's a required field or not and if it will screw things up if you just delete it.