From Wiki

• Just as each post has a karma button, each post also has the reverse.

• You do not have the opportunity to send another poster bad vibes, with an explanation of why you are sending them.

• But you do have the ability to alert the moderators to a bad post (spam, bullying, etc.).

• You can do that by clicking on the little exclamation mark next to the karma button near the bottom left hand corner of the post.