The type of people who push boundaries to the max, they know that they are doing wrong but they don't know your reaction to the wrong-that's why they do it.
The reaction to certain types of behaviour should always result in a major change in the dynamic of the relationship. If the change is made instantly then it is possible to remain friends-at a distance-while the person seeks help but if the bad behaviour is ignored, or forgiven without consequence and then escalates, eventually the only solution is to cut off all ties because this becomes the only way that you can help the person, whilst protecting yourself.