South Sydney and NRL officials reiterated their support of Mitchell on Friday after a fan targeted him with a racist slur during Thursday's 16-10 loss at Penrith.
Vision from the tunnel at halftime showed several South Sydney players, including Cody Walker, pointing to a fan in a Sydney Roosters jersey and alleging they made the comment.
The fan is believed to have left the scene after making the comment but a man who was sitting with them - said to be the fan's father - was ejected by security.
A Biripi and Wiradjuri man, Mitchell has faced racial taunts in the past and in April 2021 two men were charged for sending him abusive social media messages.
Cleary, who has played alongside Mitchell at state and international level, was stunned by the incident.
"It's just disgusting," the Panthers halfback said.
"There's no point in a game where you should feel like you have the right to say stuff like that to a player.
"I hope they never come back here. They're not fans if they're saying that stuff."
Cleary's comments came after South Sydney coach Jason Demetriou also made clear he had been taken aback by the incident.
"This is not something Latrell just cops on the footy field, this is something he's had to deal with his whole life. All Indigenous people do," Demetriou said.
"I can't get my head around it. I don't understand in Australia how anyone can be raised like that.
"There has to be hard-and-fast rules. If anyone comes close to getting racial, they are completely ruled out of our game."
Greg Inglis and Trent Robinson have also backed the call for a life ban, while South Sydney chief executive Blake Solly said he was advocating the strongest possible action.
The issue went beyond rugby league on Friday, with super-welterweight world-title hopeful Tim Tszyu opening his press conference by throwing his support behind his No Limit Boxing stablemate ahead of Sunday's fight against American Tony Harrison.
"I just want to say a shout out to Latrell," Tszyu said.
"There's no need for racism in any sport and let's keep it professional. Good on (him for speaking up).
"It's not good for sport, for any sport.
"There's no need to bring any of it, or politics or religion, stuff like that.
"We're just trying to keep it professional, and the way Latrell handled it was perfect."
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