Kim Kardashian -- Unleashing Heavyweight Lawyer to Silence PR Flack

Kim Kardashian is finally firing back at the guy who has been making claims her wedding was a sham -- by hiring legal pit bull Marty Singer to put a muzzle on him ... TMZ has learned.

A guy by the name of Jonathan Jaxson has been doing a media tour to promote his book (coincidence?) and saying Kim never wanted to marry Kris Humphries and her wedding was all staged.

Jaxson claims he knows this because he did PR work for her, but a source close to Kim tells us they worked together once, years ago, on a blog ... and that Kim barely remembers who he is.

We're told Kim is particularly baffled how Jaxson can talk about Kim's wedding plans ... when she hasn't spoken to him in years.

But as part of their brief time together, we're told Kim had Jaxson sign a confidentiality agreement -- which Kim clearly feels he's violating now. Kim has now hired Marty Singer, who submitted papers to a private arbitration to attempt to shut Jaxson up immediately.

KK is claiming Jaxson has damaged her reputation with his allegedly defamatory statements and she wants a minimum of $200,000 in damages.

Singer tells TMZ, "Everything that he is saying is false and nonsense ... This guy is in it to promote himself for his 15 mins of fame."