Sometimes it happens the TFS Administrator has to solve problems caused by unexperienced (or untrained) users, and maybe the most common is to undo ‘ghost check-outs’.
Here’s the answer: Attrice Corporation Team Foundation Sidekicks. Cool. But sometimes you can’t use third-party tools, and you only have to rely on what Microsoft provided in the OOB installation.
So here’s the real answer, since Team Foundation Server 2005: fire the Developer Command Prompt, and launch the tf undo command with the proper switches.
Here there is the relative documentation.