Disk Quota Entries Delete

I am runing Windows 2000 Server with Disk Quota Management turned on.

Recently I tried to clean-up some of the quota entries. Some of the name is displayed as "Account Information Unavailable". When I tried to delete these entries, it will redirect me to a Disk Quota window where I can take ownership to files which belong to the user. However, there is no file is owned by these users. As I close the Disk Quota window, it will said "Disk space is charged to the account of a selected user. Users charged with disk space cannot be deleted".

I took all the ownership of these users' files already. Why I cannot delete these entries?