As it stands:
How do I disable commenting in my journal?
To disable commenting in your journal, go to http://www.livejournal.com/editinfo.bml and uncheck the option labeled "Enable message boards," which is found near the bottom of the page, then click "Save Changes".
You also have the option to disable comments on individual entries, while leaving commenting enabled for the rest of your journal. Using the web client, you can do this from the full version of the Update Journal page at http://www.livejournal.com/update.bml?mode=full. While posting or editing the entry, check the box marked "Disallow Comments" to disable comments on that entry only. If you are disabling comments on an entry that already has comments, these comments will then become invisible.
If you use a downloaded client, your client may also allow you to disable comments on an entry. Consult your client's documentation or menu options for more information.
If you would like to block comments from a particular person, but still receive comments from others, see http://www.livejournal.com/support/faqbrowse.bml?faqid=20.
If you would like to keep comments enabled, but want to stop receiving notification emails, see http://www.livejournal.com/support/faqbrowse.bml?faqid=62.
Thoughts? I suppose then, the title might need changing to "How do I disable comments?"
[Edited: Added suggestions from lyspeth, henningz, beginning, and ruakh. Changes are in italics.]