You can always just use NETDOM on the workstation. Type in:
netdom remove /d:mydomain mywksta /ud:mydomain\admin /pd

assword
Reference Example 8 here:
http://technet2.microsoft.com/WindowsServer/en/library/539c5381-db4f-445f-aac0-2df5448181c11033.mspx?mfr=true