Randomly banned on multiplayer world

I made a multiplayer creative world so I could play with my best friend and his brother. It was fine until after a while, my friend's brother was just banned. Neither of us banned him. He was just banned. There was a message that said that he broke "Rule 1" I put up signs with rules, but I didn't put them into settings. He didn't even brake any of them either. After a while he was unbanned, but it happened again later.