Levy absolutely mauled a smack-talking social media user named Ben recently ... after the guy had defended noted antisemite Nick Fuentes in an exchange on X.

Levy, who's Jewish, met the dude at his gym and pounded on him for several minutes -- forcing the guy to tap out.

Eventually, he got Ben to say on camera following the sparring sesh that six million people were murdered in the Holocaust -- an atrocity Ben didn't previously acknowledge.

When Levy joined Harvey and Charles on Tuesday's "TMZ Live" to talk about it all ... he said he was pleased with how it turned out with Ben -- and now, he wants others to keep the incident in mind before they hit their keyboards with similar nonsense.

"The big lesson I wanted to teach this guy -- and hopefully everybody who is watching -- is words have consequences," he said. "Actions have consequences."

"If you say something, you better stand behind it," he added, "'cause this hate turns into hate in the real world. So we need to cut it off."