Selena Gomez seemingly won't turn down any invitation to diss her ex Justin Bieber in the press or public eye, but that doesn't mean he's going to follow suit.

The Canadian phenom, who is arguably one of the main reasons anyone knows Gomez's name outside of the Disney universe, is taking the high road, even if it gives him a nosebleed.

“She’s been bashing him but Justin has ordered everyone around him not to fight back,” a source told Radar Online. “He told his friends and everyone else about Selena, ‘She’s off limits.’”

The insider added, “He told everyone to leave her alone. He doesn't want to fuel the fire and he just doesn't want it to be that way with her. ‘I don’t want anyone saying anything negative about her,’ is what he told his pals.”

It's a smart move on Bieber's part considering he's been dealing with a few PR disasters as it is ... but we'll see what happens if he's ever on 'Letterman' again.