Interrupt the behavior and ask whether the conversation is solving a problem. People who have been rewarded for their irritating behavior won't quickly change. It's an ongoing process. Again, this is a lot like parenting. The most difficult problem in rearing children is that the child's main job is to try and get around his parents.
Parents have other things to do, like making a living and cleaning the house.
While consistency, as one sage said, might by the hobgoblin of small minds, it's the only way to change the behavior of that difficult employee. The odds are that, if we swapped jobs with the boss, we would irritate him or her as much as he or she bothers us now.