I'm of the same position that you've got to be able to remove users out of the database, but I do understand the need to transfer submissions from authors. However, getting rid of users with no record attached to them should also be an option.
In that case, and in my case, I have a brand new installation that I created a couple of test users to see if the LDAP authentication was working. The tests were inconclusively because the error messages were not logical with a bad password. So I went into the user and user setting table and deleted my account, the only account besides the admin, so I could try again. But alas, for some reason, my account is still hanging around the database somewhere.
So... can someone tell me all of the places in the database I need to purge all instances of a user to get rid of them? I do have a DB interface (webmin) that I can use to swiftly maneuver through the database.
I'll search, then post in another place the issues I'm having with the LDAP authentication.