Beware that NOT IN may be really slow. Sometimes - surpringly enough - its faster to do something like this:
DELETE FROM items WHERE id IN(SELECT id FROM items EXCEPT SELECT item_id FROM users)
DELETE FROM items WHERE id NOT IN (SELECT item_id FROM users)
(uses a subquery to select all the item_ids from users and then deletes the records from items where id is not in the results of that subquery)
item_ids
users
items
id
delete from itemswhere id not in (select item_id from users)