How can I mark posts as spam on my wall or the page's wall, which I manage using my application? I have read_stream, manage_pages and publish_stream permissions. I've tried to google it and to search in FB documentation, but found nothing.
update
Can I later un-spam that post via API?
you cant update a post, you just can delete it via api. see https://developers.facebook.com/docs/reference/api/post/ for more info.