There is a vexatious litigant statute that deals with multiple filings of frivolous lawsuits in pro per. It requires 5 lawsuits within 7 years to be determined adversely, other than small claims, or repeated relitigiation of same issue or motion. See CCP 391. The filing of a moton under this section stays the litigation until she pays security into the court.
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I think what you are talking about is not vexatious civil lawsuits, but maliciously inspired criminal complaints to the police and CPS. No judge is ever going to order someone NOT to report abuse of children, no matter how many past false complaints were made. The ONLY solution is one that prosecutors absolutely refuse to utilize: prosecute HER for these false police reports. Otherwise, she has no motivation whatever to stop her malicious campaign against him. I wish I could offer you a solution, but I have not found one yet for this recurring problem. Maybe the government will notice the problem when instead of your nephew it is the son of the chairman of a legislative committee.
The worst part is that not only does she have virtual immunity for her criminal acts, but it is FREE, because our tax dollars fund her hate campaign. your nephew has to hire a lawyer. She does not. It's a terrible dilemma.
The only approach I can suggest to him is to collect up a stack of all the documentation of her false complaints; the court rulings and investigative reports finding that they are baseless, and make several copies of the package. Keep one in the car. One at home, and one at work. So that when the next Gestappo assault occurs, he can just smile, hand them the stack and say "When will you people learn?"
Mr Dinday is correct that this is not a vexatious litigant situation which has to do only with civil suits. These are criminal charges and as Mr. Dinday said no one is going to order her not to file them. One hopes that after a time she will lose all credibility with the police and the courts.