Interestingly, Valve does not ban till about a month after the infraction, presumably to detect whether it is a false alarm, but also to keep hackers in the dark as to which methods can be detected and which can't.
As for infallable software, it is fairly easy to write software that avoids false positives. Look at virus scanners. If an antivirus reports a file, the odds of a false positive are very low (which means the odds for a false negative are, unfortunately, higher).