Like, earlier i told you that in this game i had downloaded, they had the maps and models in dll files so there was nothing i could do to change the maps or models.
That must be the most idiotic thing I have ever heard of. Packing resources into dlls in the hope that they won't get modified at the expense of good software development practice is a crime against the user as well as the developer who has to put up with the weird antics of people who come up with such ideas.
If they are able to play the game on their own PC disconnected from the internet then I'm very sorry, but there is ABSOLUTELY NOTHING you can do to prevent them from changing the way the game looks or functions. You really shouldn't care either because as long as they keep it to themselves they don't bother others do they?
If on the other hand you want to play the game in multiplayer, the typical method used by many games out there is to keep a copy of the game state on every player's computer and perform the same calculations every frame on the game state. If a player's state deviates from the other players' states then his game goes out of sync and he is kicked or whatever. This would indicate to the other players that he manipulated his game in some way.
So to keep it simple. In single player, don't give so much effort, it isn't worth it. In multiplayer, there are methods, one of them I described here.