This is fundamentally not possible. It's an arms race which the game writer cannot win, because, ultimately, the code is on the host's computer. It's an analogous situation to DRM, interestingly enough. The absolute best you can do is make it difficult for someone you don't want hosting to host - difficult enough that they won't bother. For example, if your game goes through a challenge-response protocol with a server you control, using some sort of date-dependent hash or whatever, they'll have to decompile the game to get around it (effectively).