When a list field definition is "sealed" or "inherited from base type", for SharePoint 2007, as I know, it's impossible to remove it from list. What we can do is to "hide" it from "view form", "edit form" and "new form". But it is still there in the list settings and list view settings page.
However, in SharePoint 2010, it becomes quite easy to remove this kind of field from list. Because Microsoft doesn't seal the field definition any more. With the help of "SharePoint Manager 2010", we can change the field attribute "Sealed" to false (then click "Save"); then change the field attribute "AllowDeletion" to true (click "save" again). After that, you can see the "Delete" button appeared in the list field configuration SharePoint web page.