(Sort of) 'official' definition:
http://en.wikipedia.o.../Troll_%28Internet%29
While the Ed doesn't really approve of multiple usernames, it's probably not enough reason for a particular contributor's posts to be blocked. However some INDIVIDUALS are banned from this site. It's NOT a username which is banned, it's a PERSON. If it becomes obvious that a 'new' poster is actually a banned user, their posts are likely to be blocked because of who they are (and what they've posted in the past), not simply because they've used more than one name.