In the edit watched lists (topics, users, forums), you must currently (it seems) check each item individually that you want to delete. I've had some users ask if there is a way to delete all from the watched lists and I can't see any easy way.

In the next version, I'd like to suggest a "Check All" and "Uncheck All" buttons like you have for PMs. So that a person could then delete an entire page of watched items at once.