How can I delete a single curve?

From Plot->Chart Settings for Active Plot..., click on the curve(s) to be deleted and click the delete button or delete key.

You can also do it from the List of Curves Toolbar (View->List of Curves Toolbar) as shown below.

Mark the curve(s) you want to delete by clicking on the curve name(s). Use Shift or Ctrl if you want to mark several curves. Then use the Delete key to delete the marked curves.