Nice and usefull one:
1. Open Command prompt
2. Go to Program Files (x86)\Microsoft visual Studio 10.0\Common7\IDE (or in 9.0 if you are using VS2008)
3. run witadmin.exe destroywi /collection:http://[tfsurl]:8080/[collection] /id:workitemid
where workitemid is the id given to the workitem by TFS
You can also see the documentation here



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