"We'll treat every case on its merits but where there is violence against women it sits at the top of the pile.
"It's a very clear message for a lot of players in our sport that it's a great privilege to play rugby league, to put a jersey on, to earn a significant income which they deserve as footballers.
"But it comes with real responsibility. And if you start to step outside the crease, you're going to put your livelihood at risk. This is a prime example."
The incident between Barba and the mother of his four children, Ainslie Currie, is being investigated by police.
Barba was sacked by the North Queensland Cowboys on Friday night after leaving the English Super League to join the club ahead of the 2019 season, and Greenberg threatened the Dally M Medal winner with a life ban from the NRL if he was proved to have been violent towards a woman.
The NRL boss said he was told in great detail what the video footage from the casino showed, and that made it a simple decision to deregister the Queenslander from the league.
Greenberg stopped short of using the words "life ban" but left nobody in any doubt that's what the penalty essentially was. He said he did not see a time "or any time in the future" when Barba would be welcomed back to the game.
The Super League said it would uphold any penalty imposed by the NRL, meaning returning to England to play is not an option for Barba and while Greenberg said he won't call for a worldwide ban, he said the former Bulldogs and Sharks star should start looking for another job.
"I can't see a time at any time in the future when he'll be welcomed back," Greenberg said.
"There's no place for him. It's probably time for Ben Barba to find a new vocation.
"We're not asking you to play, it's a privilege."
Rugby Australia said yesterday Barba wasn't welcome in its code if he was banned from the NRL. The 29-year-old has had some rugby experience, joining French giants Toulon on a $2.5 million two-year deal back in 2017 but was cut after just three months and four games.
Barba was on his last chance at the Cowboys after being forced out of the NRL in 2016 for recording his second illicit drugs strike following Cronulla's grand final win.