Hello,
the easy way is to put the field readonly, on the template.
Open the file with any code editor (Recomend: NotePad ++):
/[your-blesta-dir]/app/views/client/bootstrap/client_main_edit.pdt
After the last line of that file add this code:
<script type="text/javascript">
$(document).ready(function() {
$("#email").prop('disabled', true);
});
</script>
save the file and replace it
Hope that helps
Regards,
PV