Negative karma is supposed to be issued when you find someone's post in poor taste, or simply disagree with it. Conversely positive karma is given to good posts.
Consistently handing out negative karma to the same person or people for no other reason than you dislike them is personal, and against the spirit of the rules of the rules of this forum. It is intended as a reward or punishment, if you will, for good or bad posts, not as a personal attack against someone.