Firstly, you must have administrative right to manage workspace as shown on the photo above, then open up the developer command prompt and enter the command below
tf workspaces /Owner:* /format:xml > d:\workspaces.xml
this will export an xml file with all the details given, you then issue the command to delete the workspace base on the generated xml file
tf workspace /delete "workspace-name;owner-id"
You may refer microsoft doc for complete command