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.
