Prevent adding elements to List from Custom Field

I have a Custom CRM Field of type List/Record with its associated List that applies to Case only. Default access level is setup to "Edit" and "Store Value" and "Show In List" and "Mandatory" are checked. No other particular configuration is done on the field.

Problem is that everyone is able to add new elements to the list itself and that is not the desired behaviour. Changing the Default access level to "View" prevents the field from being modified after a Case is created which doesn't work for the current use case.

Is there any way to create a Custom CRM Field that is associated only to Cases themselves so that a logged-in NetSuite user can edit the value of the Field but only from a predefined list that can not be changed? (similar to a Case's Status or Type)