Microsoft will not sit idly by as cheaters continuously try to tear apart their highly-regarded achievement system. They have done everything in their power to keep it as balanced and fair as possible and continue to do so via an announcement straight from the Major himself:
Here at TrueAchievements, we do everything in our power to maintain that same level of fairness Microsoft is battling for. See our cheater policy here if you have any questions regarding what crosses the line - Cheat Policy
If you come across a cheater yourself, do not hesitate to report them to us, as we have a team dedicated to the removal of every last one. The link above outlines how to file a report if you are unsure how.