the wallhack is the one i hate the most though. it really ruins hide and seek/"stealth" games. even in simple deathmatches you can't hide behind corners or in crates. the wallhack is by far the hardest to detect/fight against as an admin. the only way of patching it i can see is just removing the other player objects from the client if they can't be seen or something, and that would probably turn out really bad.
I caught one once by turning myself invisible and standing in a really obscure corner of the map and watched him show up and stand there confused for a second before shooting to check if I was really there. I think an even better way if you have someone you suspect of using it would be to hide inside in some inaccessible area and spycam them. Watch them run around the nearby area looking desperately for the button to open a secret tunnel.
