Steps For Permanently Removing Work Items From Team Foundation Server 2010

Prerequisite:

1.     User must be a member of the Team Foundation Administrators security group or the Project Administrators security group for the team project collection.

2.     WITADMIN utility should be present in C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE

Steps:

1.     open command prompt as administrator and browse to C:\Program Files\Microsoft Visual Studio 10.0\Common7\IDE

2.     run witadmin destroywi /collection:<collection url> /id:<id>

Please refer this link for more detail.