You can write (although solicitors letter probably more effective) telling her that unless she ceases and desists saying whatever it is that saying or posting online or wherever, you will apply to court for an injunction prohibiting her from mentioning you in speech or print again and that you will also ask the court to award the legal costs of the action against her.
You can also threaten an application for compensation in respect of damage to reputation.
Often, just the threat of legal action on its own is enough to put a stop things.
Can I clarify anything for you?
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