Having a conversation with the person would be of great help. Try to explain yourself in a polite manner, make the person see reasons why he or she should not act in a certain way.
For example, if someone you know is in an abuse relationship, let that person know that he or she deserves better than what he or she is getting.
If they realise this and they are unable to help themselves, help them seek for help. Maybe counseling or becoming a member of a support group might help them get out of that habit.
I think it's important to help people nurture their best qualities and never allow someone to become less than what they are meant to be. For example, if a woman who has real potential, is dating or married to someone who is abusive and will not allow her to be herself.
I would try to make her see that she could be doing so much more with her life, instead of being stuck in a bad marriage or a bad relationship with someone who will not allow her to grow. Also, if someone is a generally good person and they start to do harmful or destructive things, I would do all I could to remind them who they are and that their behavior has turned them into someone they are not.