You can delete a group in a catalog provided it contains no elements.
To delete a group in a catalog:
Select File > Catalogs > Catalog Manager, or right-click an element in the catalog panel and select Catalog Manager.
Select the appropriate element type from the Element drop box.
In the Select a Type window, select the group you want to delete. Note that the group can only be deleted if it contains no elements.
Select Catalog > Delete Group, or right-click and select Delete Group.