Here are some power tips for doing things to your vBulletin database directly from the MySQL command line.
Always backup your database before editing it manually, or you'll be sorry...
Delete Guest Posts
delete from post where userid = 0;
Delete Soft Deleted Posts
delete from post where visible = 2;
Delete Soft Deleted Threads
delete from thread where visible = 2;
Delete Thread Redirects
delete from thread where open="10";