I’m assuming that you are using ACF4 because you indicate that you cannot add field to the user registration form. ACF5 can add custom fields to the user registration form.
ACF5 does not however have a way to add a disabled option in the select field.
I think that this can be simulated by setting the field to required and to not allow null values and then using an acf/load_field filter to insert a null entry into the beginning of the selections https://www.advancedcustomfields.com/resources/dynamically-populate-a-select-fields-choices/. While the first item would be selective, validation would fail submitted with it selected. I haven’t tested this, but it seems like it should work.
