Hi Ben,
I know this is a slightly old post but thought it might be useful for others to answer you.
Take a look at this section of the docs from an older release – https://docs.openiam.com/docs-4.1.14/html/administration/sysadm/page-templates.htm
Login to the WebConsole, go to Page Templates under the Administration menu. Find the Default template and edit it. The default template governs what end users on the self-service portal see. Pick the fields you want to prevent the end user from editing, click edit and untick the Is Editable box and save the field.
Neil