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.