Ability to deactivate custom field options

The custom fields are great. However, with the radio and select "field types" we're unable to remove a selection from the settings without it affecting previous entries.

e.g. we have a custom field called Course Source. We use this field to identify how the student heard about the course prior to enrolment. The Course Source is a drop down that provides a list of all of the sources we're marketing and tracking. At the moment there's no option to be able to remove or deactivate one of the selections without it affecting any data already input into the system. 

We want the data to be quite granular so we have quite a lot of options. As this field is not searchable and we can't deactivate or remove any of the previously used selections this makes the custom field more difficult to use and decreases the data input accuracy due to human error.   

I'm suggesting any or all of the following options (in order of preference):

1. Be able to deactivate previously used selections/option 

2. Be able to search for custom fields in the same way that other field types have been updated recently

3. be able to delete previously used selection/options without it affecting previously input data.