I agree with Lehsyrus. If it were me doing it, I'd encrypt some of the games files on disc and therefore force the user to run the antihack loader (and login) in order to play. Then let the loader inject the antihack module into CA which makes it harder for the average user to terminate it.
And if the game/the games real hackshield hooks WriteProcessMemory() and so on, you already know that if they use hacks, they will inject them. So you could just fetch a list of currently loaded dll's in the process and compare them with a "normal list" for the game, if anything looks odd -> upload the info including hack name and time for occurence to a mysql db.
Naturally they could reinstall the game in order to bypass the loader. This is however simply solved I guess, the loader should save whenever they connect to the database and if you see them ingame but the loader haven't been running for weeks, it's a sure bet they done something fishy and you probably want to kick them out of the clan.