You can delete Packages from your Catalog individually or you can delete multiple Packages at one time using the Delete button. You can select multiple Packages in the grid use by holding down Shift (to select a series) or Ctrl (to select outside of a series).
You can also right-click to delete, or use the keyboard shortcut Ctrl+Del.
There is no "undo" in SI. Once deleted, there is no recovering these Packages unless you restore a backup of your SI server.
Deleting Packages from the Package Explorer does not remove the packages from existing Projects.