How to remove rows from a table A that has no relationship with table B?

Here's a way to do it:

DELETE FROM TABELA_A where TABELA_A.FK_ID not in (select FK_ID FROM TABELA_B)

Using NOT EXISTS

DELETE t_a
  FROM a t_a
 WHERE NOT EXISTS(SELECT 1
                    FROM b t_b
                   WHERE t_b.fk_id = t_a.fk_id)

Using JOIN

DELETE t_a
  FROM a t_a
  LEFT JOIN b t_b ON t_b.fk_id = t_a.fk_id
 WHERE b.fk_id IS NULL