There is no way to ban a specific person from viewing your public journal entries. To restrict who can see an entry, you must alter its security level as described at http://www.blurty.com/support/faqbrowse.bml?faqid=72 . If you leave an entry public so that it's visible to people who aren't on your friends list, anyone can read it.
Banning someone from reading your journal when they're logged in under a certain username would be pointless: they could still read your public posts by simply logging out. Even if you could also restrict access to logged-in users (not a currently available security setting but one that has been suggested), the person you want to ban could easily make another account and continue to view your public posts.
If you could ban someone by their IP address, you would end up banning other people too, and the person you wanted to ban could still view your journal from a computer that connects to the internet differently.
Since any attempt to block someone from reading public posts could be circumvented, Blurty does not offer such an option. If someone can find your journal, the only way to stop them from reading your posts is to make the posts non-public.
You can, however, ban an individual user from commenting in your journal, as explained at http://www.blurty.com/support/faqbrowse.bml?faqid=34 . If the user creates new accounts to get around that ban, you will be aware of it, and you might be able to take further steps to prevent any abuse.
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