To leave the Rich Text editor, you will have to re-enter the Update Journal page by clicking on a link to it.
This isn't true, because of the use last state thing. Until a link to switch back is put on update.bml, this FAQ should link to http://www.livejournal.com/settings/?tag=editor.
Related request: http://www.livejournal.com/support/see_request.bml?id=587894