Hi Evy

There is a setting to lock fields under Site Admin > Plugins > Authentication > email-based Authentication and also manual accounts.

I locked the city field hoping that this would mean that this field wouldn't be able to be populated when the signup form is used. What happens is the fields can be populated but not changed by non admins.

If you could make these fields to not show then you can create custom user profile fields and select whether this is locked  or displayed on the signup page. (Site admin > Users > Accounts > User profile fields)

To make these fields hidden would probably require and alternate login URL with a signup form created with field you require.

regards