I think your question involves issues of defamation of character and 1st Amendment liberties. First, any person has the right to state his/her opinion about any subject. Remember, the 1st Amendment of the U.S. Constitution provides that all persons have the right of freedom of speech with certain limitations. For example, a person cannot lawfully shout "fire" in a crowded movie theater when there is no fire. Those words are deemed fighting words and could cause severe harm to people trying to exit to escape a hazardous condition that does not exist. However, if a person is making false claims and conveying these false statements to other parties, that person is committing an act of defamation of character and can be sued in a civil court. You may even seek an injunction stopping the person from spreading these false statements. Be advised that the "truth" is an absolute defense to defamation and the burden is on the defendant to show that his/her statements are true.
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