Calling someone a thief without proof and publishing that information would be damaging to that individual's reputation. The same could be said if you accused someone of a child sex crime.
Some may argue that you have freedom of speech and can say whatever you want. In the United States, you have a right to the freedom of speech, but one thing that you cannot do is hurt another person's reputation as a result of making false claims. This is known as defamation. The term covers any statement that hurts another person's reputation. If you publish or write the claim, then it is called libel. If you speak a hurtful statement, it is known as slander.
Defamation law has to allow for some freedom of speech. There is a fine line, though, as hurting a person's reputation could cause them to suffer in a way that harms their ability to work or have normal relationships. If you have been hurt by defamation, you can file a claim against the person responsible. You will need to show that someone made a statement against you and that it was published. The statement must have caused you financial injury or damage to your reputation. The statement must have been false and the statement must not fall into a privileged category.
If you can show that the statement was spoken, typed or expressed in a way that harmed you, then you have the basis of a solid case. Show that you have suffered as a result of the statement. You should also show that the statement was false. If it was true, you don't have a claim against the other party. If you have the evidence you need, then your attorney can help you begin your case.
Source: FindLaw, "Defamation Law: The Basics," accessed July 25, 2017