To ban a user:
* After you delete a comment from any LiveJournal user, a checkbox shows up that lets you ban them. If that is selected, that user will be unable to leave further comments in your journal.
* If you want to ban a specific LiveJournal user without having to delete one of their comments, you can ban them using the Admin Console (http://www.livejournal.com/admin/console/). First, make sure you are already logged in, then go to the console and enter this command:
ban_set < user >