Notices by Catgirl (jte@the.resize.club)

Easy answer: You don't. You'll just have to deal with the fact that individual game clients can say, do, and display whatever the heck they want, and program your networking accordingly to do a reality check on literally anything and keep strict control over "secret" information like players in other rooms across the map.

Hard answer: Make the game only run from an encrypted install on boot disk, as its own OS?? Not even then..