In California and in many states if your employer makes false statements about you regarding your trade or prefession is it defamation per se. Unlike other types of defamation, with defamation per se you often do not need to show harm. Defamation about a person's trade can harm their ability to earn a living. So, states take extra steps to protect employees in such circumstances.
There's no law that prevents a former employer from saying bad things about you; however, if the former employer states anything that is not true, the former employer could be liable for slander (or libel, if the statement is placed into writing).
If someone threatens a bad action if you don't pay money, that could be extortion (which is a criminal act).
Because there is a fairly complex interaction between civil and criminal law in this instance, and because each element is so very fact sensitive, I strongly recommend you consult with an attorney to develop a plan of action.
Of course, I'm not your attorney, and this isn't legal advice. My intent is to provide you with some information so that you may find a competent attorney and understand the issues.
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