A lawyer repping celeb boxing promoter Damon Feldman has fired off a cease and desist letter to Farrah threatening to sue her ass if she continues trash talking him and the event ... slated to go down Saturday in Atlantic City.

The lawyer, John A. Gallagher of the Gallagher Law Group, calls Farrah's bashing defamatory. In the letter, obtained by TMZ, Gallagher says Feldman kept his end of the deal ... which included paying Farrah's fees up front, providing training and equipment.

What's more ... the law firm says it was Farrah who bullied promoters with unreasonable and unnecessary demands. Promoters gave in ... with a suite of rooms for her posse, but did so as a courtesy to maintain the relationship. It was NEVER part of the deal.

Farrah's manager, David Weintraub, tells us, "This letter is utterly hilarious coming from these "janky promoters" pretending to be a boxing organization. We wish them lots of luck trying to enforce our rights and ability to speak freely about the absurd way they handled their amateur business over the course of this deal."

He continues, "Bottom line, they have no money, they have no pay-per-view deal, they aren't selling tickets and couldn't even afford proper travel, accommodations or security for my clients, hence that's why they are in breach and she isn't fighting. Enjoy your flop fight Saturday."