Currently, InsaneJournal does not provide any means to delete all of the entries from your journal with one command. To do this, you must delete each entry individually as explained at http://www.insanejournal.com/support/faqbrowse.bml?faqid=3 .
However, the process of deleting all of your entries can be done more quickly using a downloaded client, because using a client avoids waiting for pages to load on the Internet. For more information on your preferred client, consult its documentation or its menu options. See http://www.livejournal.com/support/faqbrowse.bml?faqid=158 for information on downloading a client. (Note: Not all LiveJournal clients are suppported by InsaneJournal)