A Formal Demand, Not a Court Order
A cease-and-desist letter is a formal written demand that someone stop a specific activity — in this context, publishing or continuing to publish defamatory content. It puts the recipient on notice that you consider the content defamatory and are prepared to take further action if it isn't addressed. It carries no independent legal force on its own; it's a step, often before litigation, not a substitute for it.
Realistic Expectations
Often Effective When
The poster is identified, has something to lose (a business, a professional license, an employer who'd be unhappy to receive a copy), and may not have understood the legal exposure before receiving the letter.
Less Effective When
The poster is anonymous and can't be reached, is judgment-proof (nothing to lose), or is emboldened rather than deterred by a formal-sounding letter.
An attorney should draft or at minimum review a cease-and-desist letter — a poorly drafted one can overstate your legal position, and some recipients forward these letters to their own attorney or publish them publicly, so it should be written with that possibility in mind.