You can use this SQL query to hard delete all 'soft' deleted posts:
Code:
DELETE FROM `xf_post` WHERE `message_state` = 'deleted'
Obviously you may not want to do them all so you might want to add other criteria into the query.
Or you may just want to view your deleted posts which would be:
Code:
SELECT * FROM `xf_post` WHERE `message_state` = 'deleted'
EDIT: By the way, I'm not certain if there's other things you'd need to do to "clean up" after hard deleting posts...