You could sue them for something maybe harassment or defamation but based on your brief statement, I am not sure what your damages, if any, might be. If they are violating Facebook's Terms of Service, you can report them to facebook. More likely any standing to file a lawsuit may reside with your father.
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One of the best things about the Internet is that everyone has a voice. One of the worst things about the Internet is that everyone has a voice.
I often counsel clients that there are trolls and idiots on every forum and social media site. My best advice is not to engage in communication with them - it only adds fuel to their fire.
If your father is truly being harassed (and your facts posted don't seem to support a cause of action) he can try to get a restraining order. That could end up costing him thousands and not even solve the problem.
Sorry I don't have better news for you. Deep taking the high road with your posts and leave the trolls alone.
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