Sebastian was the name of a poster who was very active in the early stages of the Miniclip Club Penguin Forums.
He was originally banned for being under the age of 13, but ended up making a habit of creating massive amounts of alternate accounts that he used to post with. While always promising to "be nice", sebastian never quite lived up to the promise and was promptly banned on the spot far too many times.
Because he had made so many accounts with the intent and announcements to "attack" the forums, for years anytime there was foul play the community almost always puts initial blame on sebastian, however Zixty8 is now used as a scapegoat. Because of his infamy in this field, his legacy has been the subject of many jokes including the fake hack-job carried out by Admin X.