Don't manually delete the user GUIDs in the DB.  This will cause all sorts of problems.  When you delete a user in Elgg, it recurses through and deletes all data associated with that user.  Elgg can't do this if you change the database manually.

If someone has your admin password you should create a new password right away.

Deleting the user who owns the blog posts will delete all the posts.  Because your database is corrupted now, I wouldn't trust that the admin is the real owner of the posts.  You'll need to dig into the database to find the entities that match the GUIDs of the blog posts and check the owner GUID.  If the owner GUID is the administrator, someone probably has your admin password.  If the owner GUID  is a user that you deleted from the DB, you will need to manually delete all the blog posts via Elgg.

Again, don't delete users manually.  It corrupts the database.

Brett,

You mean manually at the database level? So we can be confident deleting in the admin deletes all data from a user? That's a big step forward and you can confirm this working many 1.7 sites? Great work!

I delete spam users regularly, always in the admin. Their posts are deleted too.