[Andreas]

Simply removing the input fields from the templates does not prevent users from changing their E-Mail-Adresses, it just makes it a libble bit more inconvenient as they will have to manually construct the required POST.

Add this Plug-in on hook profile_updatepassword_start

PHP Code:

if ($vbulletin->GPC['email'] != $vbulletin->userinfo['email'])
{
    standard_error('Sorry, but you are not allowed to change your E-Mail address');
} 


and you're done (though you might still want to remove the <input>s to avoid confusing the users).