Protocol for legitimate Slander, defamation, intentional infliction of emotional distress, intentional interference

As Sole legal custodian/Parent I have found myself in the middle of child custody case for my 3 kids ( 12,13,17) . My 17 year old is not my ex husbands child, & after 17 years her 'presumed' father remembered her name? As this family court issue is still in process this man gets upset that I didn't return his calls and just bad mouths me as a mother all over my Facebook page. He went so far as to threaten me and my aunt in a post on my Facebook! My children have all seen these, as well as social workers etc.. involved in family court case. What am I supposed to say to my daughters when they've read the trash he says about me as a mother and I am a loser and I don't care about my kids, because if I did I would return his calls, then he will get off my Facebook. I didn't reply to any of it.