Hi @richardmayo ,
You can configure the registration form to restrict users from modifying their email addresses.
Please follow the steps given below,
Step 1: Initially we need to Create a Content snippet, set the values as shown in the Screenshot,
Name: Account/Register/PageCopy
Type: HTML
Step 2: Then, Paste the code given below in the Value(HTML) Section
<script>
$(document).ready(()=>{
emailTextBo = $("#EmailTextBox").val()
if(emailTextBo != "" || emailTextBo != null){
$("#EmailTextBox").attr('readonly','readonly')
}
})
</script>
Please refer to the Screenshot given below,

Step 3: Save and sync the Website,
Finally, In the registration form, users cannot edit the email field as it is locked or set to read-only.

Thanks!
Inogic Professional Services Division
Power Platform and Microsoft Dynamics 365 CRM Development – All under one roof!
Drop an email at crm@inogic.com
Services: http://www.inogic.com/services/
Power Platform/Dynamics 365 CRM Tips and Tricks: http://www.inogic.com/blog/