When you do ner.teach, it should just highlight one entity at a time, rather than all the entities in the piece of text. Is it highlighting multiple spans for you?
When there are (correctly) no entities I would answer accept. See this thread for a discussion of when to accept or reject. tl;dr: if you’d be happy with your model returning those spans, accept, and if you’d not be happy, reject.
Hope that helps! (Just chiming in while @ines is off being awesome)