Cheating detection can only be achieved statistically (it's kind of like proving a random number generator isn't random).

It makes it easy to achieve great accuracy in the long run, but hard to be accurate in the short run, because you don't want false alarms. So 99.9% of cheaters may get banned after a few games, but since they keep on coming up (it's not hard to simply register another account, after all), the frustration they're causing is always going to be there.