On request, I made this extension similar to my StaticPassword extension - it even works the same way.
Usage is simple - enable the plugin after selecting which usergroups will be prevented from email address changes (use CTRL or Mac Command key to select multiple). When saved, the users in those groups will not be presented with the email fields when editing their account details. Additionally, it prevents submission of email addresses within the form data (just in case you get someone who thinks they're smarter than the plugin).
- Restrict based on user group