[WorldCraft]

Go to the user's profile and click "View Blog Entries"

On the list of blogs, click the Inline Mod dropdown and choose Select All

Scroll to bottom and in Moderation dropdown, delete and choose method of deletion

Unfortunately, you will have to do Select All for each page. If there are truly 10,000 blog posts, there may be another way (via SQL query or some other method. I would advise you seek vBulletin's official support for that though as deleting directly from the database can cause major issues)

Once you do get them deleted, I'd advise you up your anti-spam measures. Question/Answer verification is one of the best methods vB has built in. Pick at least 5 good, but simple questions (not what is 2+2). Also, there are a variety of great spam protection products here such as Spam-o-Matic which help block spambots from even registering in the first place.