I think...they monitor our playing dude..
there is something called CAV (combat arms viewer)
I think nexon is recording in a way the stuff your computer do as you play combat arms, and if they found that you hack... bye bye, acount banned :P
Other way is if you was stupid enough to usea opk hack... its 100% sure that someone will take a screenie and report you.
OPK, Aimbot(head), fly,and speed hacks are the most common reasons of ban for stupid hackers -.-
because those hacks are higly noticiable for every other fuckin player in the fuckin room.