Harrassment and deflimation of characters?

my half sister signed over legal custody to my other sister( due to legal trouble with older son) 10 years ago this august. but the birth mother constanly calls, emails, me? Threating, accusing me of doing it all. She has other of her family members do the same. about every year during the summers months this starts. now she has posted degrading remarks on face book of all places that i am a child thief. its really ridiculius . i was estranged from her long before this happened. i try not to respond, screen my calls and ask them to just leave me all. I can not take it again this summer and they are just starting. Is there anything legally i can do to make her just leave me alone