Do not respond or message unless you feel its harrassment that you may report to the police. If that is the case only once tell that person to stop harassing you or you will go to the police. Block them. Its goading and tempting to get into it on social media but then your business is out for your friends to see. Its also tempting to message them and argue but it isnt worth it. If they were a friend they wouldnt do that to you. I regularly purge friends that either argue with me or arent really people I would want to talk to. In fact i have about 80 friends and I only have people I would actually speak to in real life. Block them.