tf undo /Projects/Project1/Controls/FileName.cs"
Here the Workspace and the User are the ones who locked the file. This information can obtain from source control explorer. If you want to find list of files locked by a developer use the 'tf status' command.
One important thing to remember when using 'tf undo' command is, you may have to use server name as "http://…" if you are executing command from a developer box. Otherwise you will get the ‘cannot find server’ error message for server name as well as for IP address though you are in the same domain, etc. By using server name as a URL it will uses web services to invoke the command.
You can read more detail about “Tf Command Line Utility Commands” from MSDN here.