You would have to make similar changes on the server (or on some fraction of clients - depending on how the game is organized). Which usually means some kind of packet editing or finding ways to insert code remotely. Also depending on how the game is coded, i.e. how it handles certain actions, manipulating the checksum control (if any) could be helpful.
When we introduced "Vietcong Anticheat" the idea was for all players to download the software, which would block a certain critical cheat while unlocking super powers for all.