You can delete multiple words or categories and its sub folders and words.
Tap the pencil icon to switch to Edit mode.
Select multiple items (messages or folders) by tapping over the buttons.
Tap the Delete icon on the right edit panel. You will be asked for a confirmation to delete.
Tap Delete to confirm the deletion. All the selected items will be deleted. If you have selected a folder, all the words and sub folders within that will be deleted.
Note: The deletion is final - once deleted, you can restore the deleted items and there is no Undo feature.