A thread on the Steam Forums shows that Valve is banning those who are using programs to game the holiday sale's achievement system. Apparently a good portion of the bans relate to people farming coal. Coal give users an entry into a big game pile giveaway, but it can also be combined to claim prizes in short order. But the idea behind the contest is that users ear rewards by doing certain things in the games they own. Some users have found ways to game the system, but Valve has caught on.

The best advice to those exploiting the system comes from the guy who created the thread: "A warning to Coal farmers. I would stop."

We do not know how many accounts have banned because of coal farming but we think it is more than a handful judging by the length of the thread.