Deleting standard units from map editor

[PeterLake]

How do you delete a standard unit (e.g., an emitter or redon) from a custom map? I don't see a "delete" button on the UI, and delete key on keyboard doesn't seem to work.   

[Karsten75]

hover your cursor over the unit, but don't click to select. Then delete.

[chaotea]

You use the delete key while hovering over it.