This is about blog posts, not forum posts

Backup your mysql before doing any following mysql commands (mysql commands can be done from within mysql manager like PHPMyAdmin, "SQL" tab)


Disable vbulletin blog:


AdminCP/Plugins & Products/Manage Products/vBulletin Blog (Disable)

How to delete all existing blog posts of vBulletin


TRUNCATE TABLE blog;
TRUNCATE TABLE blog_subscribeentry;
TRUNCATE TABLE blog_text;
TRUNCATE TABLE blog_summarystats;
TRUNCATE TABLE blog_views;

Delete only blog posts older than certain date:

DELETE FROM blog WHERE dateline > '1338750243'

dateline number is the fist SPAM post dateline number (find in PHPMyAdmin by browsing blog table..

Manual deletion:

Settings -> Options -> vBulletin Blog: Entry Options -> Maximum Displayed Entries Before Page Split
And then browse blog and delete entries