Dota 2 Bans 40K Over Matchmaking Abuse

A tweet from DOTA 2 reveals Valve has banned over 40,000 accounts for abusing matchmaking in Dota 2 (thanks PC Gamer via Yiddish). They don't offer details on the problem, but the Dota 2 Reddit has frequent posts about high level players creating smurf accounts and making life miserable for lower-level players as they climb through the ranks. Word is these are Steam game bans, as well as simple matchmaking bans:
We have banned over 40,000 accounts for players who were found abusing matchmaking. These bans will now appear as game bans in Steam as well as being matchmaking bans in Dota 2.