These are users that have chosen to opt out of external sites like Project-GC and third party apps, usually this is due to privacy concerns. You can still see them on Geocaching.com so it's usually pretty easy to figure out who is who by comparing your number of logged caches from different COs against the list, but it requires some manual work. There's no setting for removing them from the tables and personally I think this would be a bad idea since it gives you incorrect data.