It's been a while since I've used this, but I think it worked
DELETING MEMORIES IN BULK
You can also delete memories in bulk. This is done by clicking on the specific memory category that contains the memories you want to delete and then adding &multidelete=1 to the end of that URL in your web browser's address bar. This will provide checkboxes next to each of the entries in that category. Check the box next to each memory you wish to delete, then click the "Delete Selected" button. Again, this will delete the entries from your memory list, but it will not delete the entries.
More on deleting memories (http://www.livejournal.com/support/faqbrowse.bml?faqid=47)