I helped a friend via email with this last week and just noticed a similar blog post.  If you're having trouble trying to destroy an item you already deleted (to free up disk space, for instance), you may find this helpful.

http://kinnie.blogspot.com/2008/05/6-delete-vs-destroy-in-tfs.html

The thing is that you need the deletion id of the item to force the destroy. How to get that deletion id? Request the properties of the appropriate item via the properties command. After that you will be able to pass the deletion id to the destroy command and the folder should be gone forever.