Solved
Steven Seiller over at CMX solved this for me. Seems you need a sub-select
SELECT bbc_people.id, bbc_people.firstname, bbc_people.lastname, bbc_people.realmail
FROM bbc_people
WHERE bbc_people.id NOT IN (SELECT bbc_listmembers.person_id FROM bbc_listmembers WHERE bbc_listmembers.list_id = MMColParam)
ORDER BY lastname ASC
MMColParam = $row_bbc_listmembers['list_id']