Hi All- I am trying to prevent users from adding values to a custom field list. The users have permissions to add to custom lists because that’s necessary elsewhere. How can I achieve that ?

The quickest route may be to convert the list to a custom record so that you can enforce role based permissions. Outside of that, another option would be to code a User Event script on the record in question that would pull the context of the current user, and then update the fields behavior, or set a value dynamically and then set the field to display inline.