well, if you think someone has made a bad post, or has done something wrong, you can give them a red rep, which will decrease there reputation. you do so by clicking the scales above each post on the right hand side
Red reputation is a negative response given to one of your posts or comments. A green reputation is a positive response. You can give reputation by clicking on the icon representing a "scale" in the upper right of the post box.
But remember, giving red reps will also decrease your reputation! Be very careful about giving out red!! And don't do it at all until after you have received a few extra greens yourself.