At this time, it's something that needs to be created. Honestly, it's the first time that I can recall that it's ever been asked for, so it hasn't been put in yet.
There isn't a SQL command I could give, as it would have to be a script that had some logic to it in order to delete things, reassign posts to new parents, etc.
As for deleting all replies in a post. You can edit one of the posts and then select move post & replies. That might not catch all of them, as some may be a reply to the original topic, but should help get a few at a time.
