A hulking American bodybuilder has copped a tidal wave of abuse after he repeatedly face slapped a fan with mental disabilities during a meet-and-greet event.

Rich Piana, a self-confessed steroid user , turned into an ugly schoolyard bully when he confronted a fan who he believed had sabotaged his social media accounts.

Jason Genova, himself a bodybuilding enthusiast, took repeated slaps to the face and head at Europa Games event in Orlando last week.

Genova has a substantial social media following, and as he moves in to meet Piana, the former Mr California calls him out over an " Order 66 " – an obscure reference to a Star Wars movie.

Bodybuilder Rich Piana and Jason Genova at the event in Florida. Source: Facebook

The "Order 66" occurs in the 2005 prequel Star Wars: Episode III Revenge of the Sith movie, when clones turn against their Jedi commanders and terminate them.

Genova admits delivering an "Order 66" on Piana to his online followers, who then went out and caused havoc on the professional bodybuilder's social media accounts.

Piana, heavily tattooed, bronzed and busting out of a black muscle shirt, is relentless in his badgering of Genova.

The two pose for photographs before Piana demands that Genova make himself available for a slap fight as punishment for the "Order 66".

"I feel like I gotta do something back to you to get back at you," Piana says, offering to "slap him around a little bit on camera".

Rich Piana claimed everything was "all good" after the face slapping incident. Source: Facebook

Piana then unleashes 12 quick left and right open hand slaps, rocking Genova backwards, and clearly leaving him and the crowd unsettled by the exchange.

As footage was later uploaded to Genova's YouTube account, the backlash against Piana began.

Piana later wrote on his Facebook page:

"[Jason] admitted he was wrong and said he learned a valuable lesson and is no longer going to order 66 on people. He realizes that our social media is a valuable marketing tool we use for our business and our livelihood. We shook hands and hugged and everything is good."