I recently was making some changes on a build server to TFS, but was getting an error due to a collision in mappings of working folders between pre-existing workspaces on that machine. I didnt have access to the account that the pre-existing workspaces where used under, but needed to delete them anyway. I ended up deleting the workspaces by running the following commands from the VS2005 command prompt:
To list tfs workspaces on a machine for all users run the following command:

tf workspaces /server:<tfsservername> /computer:<currentmachinename> /owner:*

To delete a workspaces for any user, run the following command (this may prompt you for credentials - you must run it as a user who has the TFS admin workspaces permission).

tf workspace /delete "<tfsworkspacename>";<username> /server:<tfsservername>