When we do determine that someone's behavior is pretty bad but not quite to the level of deserving isolation, we should emphatically state this thought process that we have gone through. Let them know that they have let us down. Tell them that if it happens again, that severance is likely. After all, sometimes it is enough for people to recieve a warning - even without their acknowledgement - to effect change. And, in the end, if we don't tell them, then we become enablers. Perhaps they will hear the same from an independent source and decide that it is time to take personal responsibility.