After you've backed up your LJ content and migrated it to Dreamwidth, use LJ-Sec to mass delete all of your old Livejournal posts in just a few clicks. I've used LJ-Sec many times before for managing my LJ posts.
Some more places with information about this issue:
Diane Duane: Some links on the Livejournal situation
Metafilter: Livejournal represents social media without borders
I've now migrated completely from LJ to Dreamwidth. I won't post or crosspost to LJ anymore, only to Dreamwidth. I'm keeping my LJ account, and I may use it to comment or contact others, but that's all. Sigh... LJ was an important part of my life for many years, but all the people are going to be there even less than they had been.
[Edited 2017-01-01 to add more information.]
This entry was originally posted at http://frameacloud.dreamwidth.org/469754.h