One problem with deleting users is dealing with their old posts.
If you delete a user who has posts in the database, you will either need to reserve his name forever (so why did you delete him?) or change the name on all his posts.
The new "myusername" should not be associated with the posts of the old "myusername."
I have seen posts been made by "guest" I assume that is because the account which has posted the thread or post has been deleted.