Can I ban someone from viewing my journal?
There is no way to ban a specific person from viewing your journal entries without using security levels (See the following FAQ http://www.livejournal.com/support/faqbrowse.bml?faqid=24 ). However, if you want your entries to be visible to the world, there is no way to ban a specific person. If you banned someone by a LiveJournal username they could just log out. If you banned someone by an IP address, you would have to end up banning far more people than you wanted, and if they chose to get around it they could do so with other ISPs, library computers, or any other method that simply involved a different IP address. If you banned by LiveJournal user and limited your posts to be read only by LiveJournal users (not a currently available security setting but one that has been recommended), then someone could easily make another account and view your pages without you ever knowing. Since banning a single user will never be secure, it is not offered.
What you can do is ban an individual user from commenting. To be effective you must turn off anonymous commenting (see http://www.livejournal.com/support/faqbrowse.bml?faqid=23 ). And then you would ban the LiveJournal username of the person ( http://www.livejournal.com/support/faqbrowse.bml?faqid=20 ). This will keep the person from ever adding a comment into your journal, and if the person creates new accounts to get around the ban, you will be aware of it and can report it to the LiveJournal Abuse team.