Essayez ceci :
ALTER SEQUENCE foo_id_seq OWNED BY NONE
alors vous devriez pouvoir supprimer la table.
Pour récupérer le "propriétaire" d'une séquence utilisez la requête suivante
SELECT s.relname as sequence_name, n.nspname as sequence_schema, t.relname as related_table, a.attname as related_column FROM pg_class s, pg_depend d, pg_class t, pg_attribute a, pg_namespace n WHERE s.relkind = 'S' AND n.oid = s.relnamespace AND d.objid = s.oid AND d.refobjid = t.oid AND (d.refobjid, d.refobjsubid) = (a.attrelid, a.attnum)