I've changed this around a little to make it a bit more flexible.

As it was if you voted for a post you then wouldn't be allowed to change your mind which I thought was a little sucky so I've set it up so people can vote for a post but can change their mind at a later date if they wish.

eg if I give good karma to a post then the person who wrote the post pisses me off I can go back and remove the karma. After that point you can no longer vote for the post.