[Mordekay]

Wrong is, that you allow non-steam users to join your server.
You want us to help you to restrict something that steamers cannot do. (really? I'm sure you cannot change my fps-settings)
Get them to buy the game and your problem is solved by itself. (or not, I didn't check that.)
And BTW. Changing clients settings is called slow hacking, which is also not supported here.
If you don't like their settings: kick them.