In the Administration Options menu, go to Search Workbench > Customizations > Topic Sets
Click the delete icon (
) for that topic set.
Click OK to confirm the deletion. The dialog alerts you if there are Search projects that use the topic set being deleted.
To delete a single topic:
In the Administration Options menu, go to Search Workbench > Customizations > Topic Sets > Your Topic Set.
Right-click the topic, and select Delete from the menu.
Click Delete to confirm.