I can't delete or reset password on accounts in Snow Leopard
This is generally really easy to clean up.

The Snow Leopard user database for all accounts on the mac is stored as flat files in the directory /private/var/db/dslocal/nodes/Default/users

Just delete the offending userA.plist and userB.plist and the accounts will be "gone" after a reboot.


boot the mac in target mode and use another mac to delete the files
boot to single user mode (or get a root shell with sudo -s) if you are comfortable in a unix shell


There will of course be remnants of the user folders and their group membership potentially left lying around.

If you are concerned about that level of cleanliness - just boot back into a normal admin user and recreate a shell account using the same short name.

You can then correctly reset the password (retaining the home folder) or delete them cleanly and totally at that point. 

Let me know if you still have a failure - it's super unlikely, but there could be a deeper problem if this doesn't work.