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NO she is not legally allowed to do that. Once she wrote the check to you for money owed to you, if she stopped payment it is considered issuing bad checks and can be actually prosecuted as a criminal offense if you follow the proper procedures. First, you have to send her written notice demanding she pay the check and any fees you incurred as a result of her stopping payment within 10 days of the letter or you will refer the matter to the DA for criminal prosecution. You must send the letter certified mail and you need a copy of the letter and return receipt (or the unopened returned letter if she refuses it) to bring to the DA with proof of her stopping payment and the DA will pick up charges against her.
You could also sue her in court for the payment for breach of contract, since giving you the check was a contractual obligation that she failed to honor.
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