You can access and turn on the new feature in your “Posting and Commenting” settings area. The button is small, but it is there towards the bottom.
You can enable comment moderation for any of your Posterous sites individually, depending on your needs. Once comment moderation is up and running, all comments are set aside until you give them the go ahead for publishing. You will receive an email when a post is commented on.
A small step for Posterous, but a huge boon to their users looking to block abusive commenters and avoid spam like the devil. The following image is of the moderation process, happy posting!