There are players who cheat to avoid limitations of the game and to gain an advantage over the best players. In most cases they tamper the cheat client and fool him into thinking they're playing legally. If you see such players, use the report button, post a screenshot, and he/she should be deleted from the game. It's impossible to avoid it at all and the developers have to implement better hacking protection tools to avoid these cases. Using a private server is even worse because the files are not up to date and are not originally developed. They're emulated and created basing on the MITM technique.